NexGard for Dogs: Everything Vets Wish You Knew

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FDA • DailyMed • NIH/PMC • Vet-Reviewed

A plain-language guide to how NexGard works, what it protects against, honest answers about side effects, who should avoid it, and the veterinarian insights that rarely come up at the appointment desk. Evidence-based. Always in your corner.

💡 10 Key Things Every Dog Owner Should Know About NexGard

NexGard (afoxolaner) is one of the most widely prescribed flea and tick medications in the United States. Approved by the FDA in September 2013, it belongs to a class of drugs called isoxazolines and works by disrupting the nervous systems of fleas and ticks, causing their rapid paralysis and death. Most dogs tolerate it well, but there are real and important caveats that do not always come up in a rushed veterinary appointment — especially regarding neurological risks, which dog populations need extra caution, and how the medication actually works at the biological level. This guide covers what the veterinary literature and FDA actually say, in plain language.

1
What is NexGard and how does it work? NexGard contains afoxolaner, an isoxazoline drug that kills fleas and ticks by blocking GABA-gated chloride channels in their nervous systems, causing paralysis and death. Parasites must bite your dog to be affected.
Afoxolaner is a non-competitive antagonist of insect GABA receptors. It binds to chloride channels in nerve and muscle cells of parasites, blocking the neurotransmitter signal that allows controlled muscle movement. Affected fleas and ticks experience uncontrolled nervous system hyperexcitation, then paralysis, then death. The selectivity of afoxolaner for invertebrate GABA receptors over mammalian GABA receptors is what makes it significantly less toxic to dogs than to the parasites it targets. The implication is important: NexGard is not a repellent. Parasites must bite your dog and begin feeding before they absorb enough afoxolaner to be killed. This means a brief window exists between a tick biting and being killed during which disease transmission is theoretically possible — the FDA and product labeling acknowledge this explicitly.
2
What exactly does NexGard protect against? NexGard kills adult fleas and controls five tick species: black-legged tick, American dog tick, lone star tick, brown dog tick, and longhorned tick. It also helps prevent Lyme disease by killing the ticks that carry it.
According to the FDA-approved label on DailyMed, NexGard is indicated for the treatment and prevention of flea infestations (Ctenocephalides felis) and the treatment and control of five tick species: Ixodes scapularis (black-legged tick), Dermacentor variabilis (American dog tick), Amblyomma americanum (lone star tick), Rhipicephalus sanguineus (brown dog tick), and Haemaphysalis longicornis (longhorned tick). It is also indicated for the prevention of Borrelia burgdorferi (Lyme disease) infections by killing the black-legged ticks that carry it. NexGard alone does not prevent heartworm, roundworm, or hookworm — those require NexGard PLUS or a separate heartworm preventive. Laboratory studies confirmed more than 97% effectiveness against American dog ticks and more than 94% effectiveness against black-legged ticks at 48 hours post-infestation.
3
How quickly does NexGard start killing fleas and ticks? NexGard begins killing fleas within 4 hours of the first dose and reaches more than 99% effectiveness at 8 hours. It kills ticks within 24–48 hours of their biting and is effective for a full 30 days.
The FDA’s Freedom of Information Summary for NexGard (NADA 141-406) reports that in well-controlled laboratory studies, NexGard began killing fleas four hours after initial administration and demonstrated more than 99% effectiveness at eight hours. Against ticks, laboratory studies showed more than 97% effectiveness at 48 hours post-infestation for American dog ticks and more than 94% for black-legged ticks for a full 30 days. Importantly, NexGard demonstrated 100% effectiveness against adult fleas 24 hours post-infestation for 35 days. The medication reaches peak blood concentration (Tmax) within 2 to 12 hours of administration. Blood levels remain protective for the full 30-day treatment period due to afoxolaner’s relatively long mean half-life of 7.7 to 17.8 days.
4
What are the most common side effects, and how frequently do they actually occur? The most frequently reported side effects are vomiting, itching, lethargy, and diarrhea — typically mild and temporary. Veterinary Partner (VIN) reports that nausea/vomiting occurs in approximately 4% of dogs and diarrhea in approximately 3%.
In the pivotal US field study cited in the FDA approval documents — 615 treated dogs across 333 households over 90 days — no serious adverse reactions were observed. Post-approval adverse event reports submitted to the FDA (listed in decreasing order of frequency on Drugs.com) include: vomiting, pruritus (itching), lethargy, diarrhea (with and without blood), anorexia, seizure, hyperactivity/restlessness, panting, erythema (skin redness), ataxia, dermatitis, allergic reactions (hives, swelling), and tremors. Veterinary Partner notes that most dogs experiencing initial vomiting do not repeat it with subsequent doses. If vomiting occurs within two hours of administration, the product label recommends re-dosing. If it occurs after two hours, re-dosing is not necessary as absorption is likely complete.
5
What is the FDA’s official position on the neurological risks of NexGard? The FDA has issued a formal alert that isoxazoline products including NexGard have been associated with neurologic adverse reactions — muscle tremors, ataxia, and seizures — in some dogs, including those with no prior seizure history. FDA considers these products safe and effective for most dogs.
The FDA’s published Fact Sheet for Pet Owners and Veterinarians about isoxazoline products states directly: “Isoxazoline products have been associated with neurologic adverse reactions, including muscle tremors, ataxia, and seizures in some dogs and cats. Although most dogs and cats haven’t had neurologic adverse reactions, seizures may occur in animals without a prior history.” The FDA also states: “The FDA considers products in the isoxazoline class to be safe and effective for dogs and cats but is providing this information so that pet owners and veterinarians can take it into consideration when choosing flea and tick products.” This dual message — real risk, overall safety — is clinically important. Research into the precise association between isoxazolines and seizures in dogs is ongoing as of the most recent publications.
6
Which dogs should not take NexGard or should take it with extra caution? Dogs with a history of seizures or neurological disorders should use NexGard with caution or seek alternatives. NexGard has not been evaluated for safety in breeding, pregnant, or lactating dogs.
The NexGard product label explicitly states: “Use with caution in dogs with a history of seizures or neurologic disorders.” In the original US field study, three dogs with known seizure histories were enrolled; two of the three experienced seizures during the 90-day period. Additionally, the official label notes: “The safe use of NexGard in breeding, pregnant or lactating dogs has not been evaluated.” This is a research gap, not a confirmed risk, but it means veterinarians generally avoid prescribing NexGard to pregnant or nursing dogs until more data are available. Dogs weighing less than four pounds or under eight weeks of age are not candidates for NexGard regardless of health status. Dogs with known allergic reactions to afoxolaner should not receive the product.
7
Is NexGard safe for herding breeds like Collies that have the MDR1 gene mutation? Yes — peer-reviewed research published in PMC confirms that afoxolaner is well tolerated by dogs with the MDR1 mutation, even at doses nearly four times the recommended therapeutic dose.
Dogs with the MDR1 (ABCB1) gene mutation — including Collies, Shetland Sheepdogs, Australian Shepherds, and related herding breeds — have reduced function of the P-glycoprotein drug transporter, making them sensitive to some drugs that cross the blood-brain barrier. A 2022 peer-reviewed study published in PMC (Drag, Tielemans, and Mitchell) evaluated afoxolaner safety in homozygous MDR1-deficient Collie dogs and demonstrated a high level of safety at 3.8 times the maximum recommended therapeutic dose. The study confirmed that afoxolaner, as a small, lipophilic, unionized compound that crosses cell membranes via passive diffusion, is well tolerated even by MDR1-deficient dogs. PetMD also confirms: “Dogs with the MDR1 mutation are not at increased risk for adverse effects.”
8
How is NexGard PLUS different from regular NexGard, and who is it for? NexGard PLUS adds moxidectin (for heartworm prevention) and pyrantel (for roundworms and hookworms) to the standard afoxolaner formula — making it a single monthly chew that covers fleas, five tick species, heartworm, and intestinal worms.
FDA approved NexGard PLUS (afoxolaner, moxidectin, and pyrantel chewable tablets) as a combination parasite preventive for dogs. The DailyMed label for NexGard PLUS confirms it is indicated for: flea prevention, five tick species, prevention of Borrelia burgdorferi (Lyme disease) infections, prevention of heartworm disease caused by Dirofilaria immitis, and treatment and control of adult hookworms and roundworms. The minimum dose provides 2.5 mg/kg afoxolaner, 12 mcg/kg moxidectin, and 5.0 mg/kg pyrantel. Important note: NexGard PLUS is not effective against adult heartworms already present in the dog — dogs must be tested for existing heartworm infection before starting. The Companion Animal Parasite Council (CAPC) recommends year-round broad-spectrum parasite control, making NexGard PLUS a practical choice for dogs in high-risk regions.
9
Can NexGard be given with other medications like vaccines, antibiotics, or steroids? Yes — the FDA-reviewed field study found no adverse reactions when NexGard was given alongside vaccines, antibiotics, steroids, NSAIDs, anesthetics, and antihistamines. No known drug interactions have been identified.
The NexGard label states explicitly: “In a well-controlled field study, NexGard was used concomitantly with other medications, such as vaccines, anthelmintics, antibiotics (including topicals), steroids, NSAIDs, anesthetics, and antihistamines. No adverse reactions were observed from the concomitant use of NexGard with other medications.” Veterinary Partner (VIN) also confirms: “There are no known drug interactions with afoxolaner.” Despite this, best practice remains telling your veterinarian about every supplement, medication, or treatment your dog receives — including flea collar use, topical treatments, or over-the-counter products — before any new prescription is started. GoodRx notes that veterinarians may still want to review a dog’s complete medication list before prescribing NexGard.
10
Does NexGard need to be given year-round, and does it matter if my dog misses a dose? Year-round monthly dosing is strongly recommended. If a dose is missed, give it as soon as possible and resume the monthly schedule. Gaps in treatment leave your dog unprotected and can allow flea infestations to re-establish.
The CAPC recommends year-round flea and tick prevention, even in colder climates, because fleas can survive indoors year-round and ticks become active whenever temperatures exceed freezing. NexGard’s efficacy period is exactly 30 days — effectiveness begins to decline after this window, leaving a gap in protection. According to the DailyMed label: “If a dose is missed, administer NexGard and resume a monthly dosing schedule.” For heartworm prevention with NexGard PLUS specifically, the label notes: “Treatment with fewer than six monthly doses after the last exposure to mosquitoes has not been evaluated and may not provide complete heartworm prevention.” This underscores why consistent monthly dosing is medically critical, not merely a manufacturer recommendation.

Sources: FDA DailyMed NexGard label (dailymed.nlm.nih.gov; afoxolaner min dose 2.5 mg/kg; Tmax 2–12 hr; 5 tick species; fleas start killing 4 hr; >99% at 8 hr; 100% at 24 hr; field study 615 dogs 333 households 90 days; no serious adverse reactions); FDA FOI Summary NADA 141-406 (>97% D. variabilis; >94% I. scapularis; >93% R. sanguineus at 48 hr 30 days; >97% A. americanum at 72 hr); FDA Fact Sheet isoxazoline adverse events fda.gov (neurologic adverse reactions; muscle tremors, ataxia, seizures; some without prior history; FDA considers safe and effective for majority); Veterinary Partner VIN (vomiting ~4%; diarrhea ~3%; no known drug interactions; re-dose if vomiting within 2 hr); NIH/PMC Drag Tielemans Mitchell 2022 PMC9543253 (MDR1 collie safety 3.8× max dose; well tolerated; passive diffusion); DailyMed NexGard PLUS (afoxolaner + moxidectin + pyrantel; 5 tick species + fleas + heartworm + roundworm + hookworm; tested for heartworm before starting; not effective adult D. immitis; safe in MDR1 dogs per PetMD); FDA Fact Sheet (seizures dogs without prior history; recommend discuss with vet); Drugs.com adverse events by frequency; GoodRx NexGard DVM-reviewed Aug 2025

🏆 10 Things Vets Know About NexGard That Are Rarely Explained at the Appointment

⚠️ All Information Below Is From Verified Veterinary & Federal Sources — Ask Your Vet Before Changing Your Dog’s Protocol

Every insight below is sourced from FDA-approved product labeling, peer-reviewed research, or clinically reviewed veterinary references. These are not opinions — they are facts that matter when choosing, monitoring, or discussing parasite prevention with your veterinarian. NexGard is a prescription-only medication and should only be started under veterinary supervision.

Critical Mechanism Fact

NexGard Is Not a Repellent — Parasites Must Bite Your Dog to Be Killed

📌 FDA Product Labeling • Often Not Explained at the Counter

🧪 What this means: Ticks will still attach. They just die before transferring most diseases.

⚠️ Fleas and ticks still land on and bite your dog

✅ Fleas die within 4–8 hours of first biting

✅ Most ticks die within 24–48 hours of biting

⚠️ Brief window before tick death means disease risk not zero

✅ Lyme disease transmission via tick typically takes 36–48 hours

✅ NexGard prevents Lyme by killing ticks within that window

The FDA’s Freedom of Information Summary for NexGard includes this precise statement: “Parasites need to start feeding on the host to become exposed to afoxolaner; therefore, the risk of the transmission of parasite-borne diseases cannot be excluded.” This is a meaningful disclosure. NexGard is an insecticide, not a repellent. A tick will still climb onto your dog, attach, and begin feeding. Only after it ingests afoxolaner from the dog’s blood does the drug reach toxic levels in the tick. In practice, because Lyme disease transmission from a black-legged tick typically requires 36–48 hours of attached feeding, and because NexGard kills ticks within 24–48 hours, the drug effectively prevents Lyme in most exposure scenarios. However, for rapidly transmitted diseases or in very high-exposure environments, this delay may carry some residual risk. Tick checks remain important even when your dog is on NexGard.

📋 Vet tip: Always do a post-walk tick check even with NexGard in place
📋 Remove any attached tick promptly — early removal reduces disease risk
🌐 FDA label: dailymed.nlm.nih.gov (NexGard afoxolaner)

Not a Repellent Ticks Must Bite First Kills Within 24–48 Hours Tick Checks Still Important

FDA Safety Alert — Know This Before Giving Any Dose

Seizure Risk Is Real — Even in Dogs With No Seizure History

🚨 FDA Fact Sheet • Product Label Warning • Ongoing Monitoring

⚠️ High-risk dogs: those with ANY history of seizures or neurological disorders

⚠️ FDA alert issued for all isoxazoline class products including NexGard

⚠️ Seizures reported in dogs WITHOUT prior seizure history

⚠️ Use with CAUTION in dogs with history of seizures/neurological disorders

✅ FDA confirms NexGard is safe and effective for the majority of dogs

✅ Most dogs tolerate NexGard without neurological effects

📞 Report adverse events: 1-888-FDA-VETS or fda.gov/reportanimalae

This is the single most important piece of information for dog owners to understand before starting NexGard. The FDA formally issued an alert specifically about isoxazoline class products — which includes NexGard, Bravecto, Credelio, and Simparica — due to neurological adverse events. The exact language from the FDA Fact Sheet: “Seizures have been reported in dogs receiving isoxazoline class drugs, even in dogs without a prior history.” This is notable because it means you cannot simply rule out the risk by noting your dog has never had a seizure. For dogs with a confirmed seizure history or neurological disorder, the label says “use with caution” — in practice, this means a serious veterinary conversation about whether the risk of untreated parasite infestation outweighs the neurological risk, and whether alternative products are available. If a seizure occurs after NexGard administration, stop the medication and contact your veterinarian immediately. Report the event to Boehringer Ingelheim at 1-888-637-4251 or directly to the FDA at fda.gov/reportanimalae.

📞 Report adverse events: 1-888-FDA-VETS (1-888-332-8387)
🌐 Online: fda.gov/reportanimalae
🌐 Boehringer Ingelheim: 1-888-637-4251 or nexgardforpets.com

FDA Safety Alert Caution: Seizure History Dogs Also Risk Without Prior History Report All Events to FDA

Prescription Requirement

NexGard Is a Prescription-Only Drug — Federal Law Requires Veterinary Authorization

🏛️ U.S. Federal Drug Law • Rx Only • Veterinary Examination Required

📝 Required: Valid veterinarian-client-patient relationship (VCPR) for every prescription

🚫 Cannot be purchased without a vet prescription

✅ Prescription ensures appropriate weight-based dosing

✅ Vet screens for seizure history and contraindications

✅ Heartworm test required before starting NexGard PLUS

⚠️ Online sources without prescription are illegal in the U.S.

📋 Prescription valid at licensed online pharmacies with vet approval

NexGard is classified as a Schedule Rx drug under U.S. federal law. The DailyMed label states: “CAUTION: Federal (USA) law restricts this drug to use by or on the order of a licensed veterinarian.” This requirement exists because professional expertise is genuinely needed — to screen the dog’s medical history for seizure risk, to select the correct weight-based dose (NexGard comes in four sizes: 11.3 mg, 28.3 mg, 68 mg, and 136 mg afoxolaner, covering weight ranges from 4–10 lbs up to 121+ lbs), and to ensure the dog is tested for existing heartworm infection before starting NexGard PLUS. Purchasing NexGard from any source that does not require a valid prescription is purchasing a product outside legal channels. Legitimate online pet pharmacies (1-800-PetMeds, Chewy Pharmacy, Costco Pet) require a valid prescription from your veterinarian before dispensing the product.

📋 NexGard sizes: 11.3 mg (4–10 lbs) • 28.3 mg (10.1–24 lbs) • 68 mg (24.1–60 lbs) • 136 mg (60.1–121 lbs)
📋 Dogs over 121 lbs: combination of appropriate chewables per vet instruction
🌐 Verify pharmacies: nabp.pharmacy (National Association of Boards of Pharmacy)

Prescription Only (Rx) Veterinary Exam Required 4 Weight-Based Sizes Heartworm Test Before PLUS

Good News for Herding Breed Owners

Collies and MDR1 Breeds Can Safely Use NexGard — Peer-Reviewed Research Confirms

🧪 NIH/PMC Published Research • Drag, Tielemans & Mitchell (2022)

🐕 MDR1-affected breeds: Collie, Shetland Sheepdog, Australian Shepherd, Border Collie, and others

✅ Afoxolaner safe in MDR1-deficient collies at 3.8× max dose

✅ Crosses blood–brain barrier via passive diffusion, not P-gp transport

✅ Well tolerated in clinical studies at therapeutic doses

✅ PetMD confirms: no increased adverse effect risk for MDR1 dogs

⚠️ MDR1 sensitivity may increase risk with OTHER drug classes — always disclose

⚠️ Discuss breed genetics with your veterinarian before any prescription

The MDR1 (ABCB1) gene mutation affects P-glycoprotein function in the blood-brain barrier, making certain drugs dangerously more available to the central nervous system in affected dogs. This mutation is common in herding breeds. However, NexGard’s active ingredient afoxolaner crosses cell membranes via passive diffusion rather than the P-glycoprotein transporter pathway that MDR1 affects. A 2022 peer-reviewed study in PMC (Drag, Tielemans, and Mitchell) specifically tested safety in homozygous MDR1-deficient Collies and demonstrated safety at 3.8 times the maximum recommended therapeutic dose with no serious adverse effects. This is reassuring, but it does not mean MDR1-positive dogs should receive any drug without veterinary supervision — their sensitivity to ivermectin, loperamide, and other P-gp-transported drugs remains a serious concern that requires disclosure at every veterinary visit.

📋 Test your herding breed for MDR1: Washington State University Veterinary Clinical Pharmacology Lab
🌐 MDR1 drug list: vcpl.vetmed.wsu.edu
🌐 PMC research: pmc.ncbi.nlm.nih.gov/articles/PMC9543253/

Safe for MDR1 Breeds Passive Diffusion Mechanism PMC Peer-Reviewed Still Disclose Breed to Vet

Pregnancy & Puppy Cautions

Pregnant Dogs and Puppies Under 8 Weeks — NexGard Has Not Been Evaluated for Safety

🐾 FDA Label Limitation • Age & Weight Requirements Are Strict

⚠️ Minimum requirements: 8 weeks of age AND at least 4 lbs body weight

🚫 Puppies under 8 weeks: do NOT use NexGard

🚫 Dogs under 4 lbs: do NOT use NexGard

⚠️ Safety not evaluated in pregnant or nursing dogs

⚠️ Safety not evaluated in breeding males or females

✅ Margin of safety study conducted in 8–9-week-old puppies at 1×, 3×, 5× dose

✅ No test-article-related changes at therapeutic and overdose levels in puppies

The NexGard label is explicit: the product is approved only for dogs and puppies 8 weeks of age and older weighing at least 4 pounds. Below these thresholds, safety has not been established and dosing would be unreliable. For pregnant, nursing, or breeding dogs, the label states clearly: “The safe use of NexGard in breeding, pregnant or lactating dogs has not been evaluated.” This is a research gap rather than a confirmed harm, but it means veterinarians typically recommend alternative parasite control for these dogs until more data exist. For dogs you intend to breed or who are currently pregnant, have an explicit conversation with your veterinarian about safe parasite prevention alternatives. The margin of safety study did test 8-to-9-week-old beagle puppies at 1, 3, and 5 times the maximum exposure dose over six treatments, with no clinically relevant effects on physical examination, body weight, blood work, or organ health.

📋 Never split or crush chewables for small dogs — the label confirms NexGard is not available in scored tablets
📋 Dogs under 4 lbs have no NexGard dose option — discuss alternatives with your vet
📋 Nursing dogs: ask your vet about program timing relative to whelping

Min Age: 8 Weeks Min Weight: 4 Lbs Not Evaluated in Pregnancy Discuss Breeding Dogs with Vet

Critical Before Starting NexGard PLUS

Dogs Must Be Heartworm-Negative Before Starting NexGard PLUS — Non-Negotiable

🩺 FDA-Required Protocol • NexGard PLUS Product Label

🧪 Why: NexGard PLUS prevents new heartworm infection but does NOT kill adult heartworms

🚫 Do NOT start NexGard PLUS in a heartworm-positive dog without treatment

✅ Annual heartworm antigen test recommended for all dogs

✅ Dogs positive for adult heartworms: treat with adulticide first

✅ NexGard PLUS kills immature larvae (L3/L4) but not adults

⚠️ Giving preventive to a heartworm-positive dog can cause a reaction

⚠️ Heartworm prevalence has increased across the U.S. — do not skip testing

The DailyMed label for NexGard PLUS states: “Prior to administration of NexGard PLUS, dogs should be tested for existing heartworm infection. At the discretion of the veterinarian, infected dogs should be treated with an adulticide to remove adult heartworms. NexGard PLUS is not effective against adult D. immitis.” Giving any heartworm preventive — including NexGard PLUS — to a dog with an existing adult heartworm infection can be dangerous. The sudden death of microfilariae (immature heartworms circulating in the blood) can trigger a systemic reaction. This is why the annual heartworm antigen test is not merely a formality — it is medically required before prescribing combination heartworm preventives. The Companion Animal Parasite Council has flagged increasing heartworm prevalence across regions of the U.S. where it was historically uncommon, making annual testing more important than ever.

📋 Annual heartworm test: required at most veterinary practices before refilling NexGard PLUS
🌐 CAPC prevalence maps: capcvet.org
🌐 American Heartworm Society guidelines: heartwormsociety.org

Test Before Starting PLUS PLUS Does Not Kill Adults Annual Test Required CAPC Year-Round Recommendation

Household Safety Alert

NexGard Is Beef-Flavored — Dogs and Other Pets Can Accidentally Ingest an Overdose

🏠 Product Label Safety Warning • Keep Secured at All Times

⚠️ Accidental ingestion risk: palatability means dogs will actively seek and eat the medication

🚫 Keep NexGard out of reach of ALL pets and children

⚠️ Multiple chews ingested = potential overdose, especially in small dogs

⚠️ Do not leave opened packages accessible

✅ Store at or below 30°C (86°F) with excursions to 40°C (104°F)

📞 Human ingestion: contact poison control immediately

📞 Pet overdose: call your vet or animal poison control immediately

Because NexGard is formulated as a beef-flavored soft chew, it is inherently palatable to dogs. This is a major compliance benefit — most dogs eat it like a treat without any resistance. However, it also creates a genuine household safety risk. The NexGard label states: “Keep NexGard in a secure location out of reach of dogs, cats, and other animals to prevent accidental ingestion or overdose.” A single correct dose is safe; multiple doses in one sitting can cause overdose symptoms. The label also notes: “Not for use in humans. Keep this and all drugs out of the reach of children. In case of accidental ingestion, contact a physician for treatment advice.” Storage should be in a locked cabinet or high shelf, in the original sealed packaging. If another pet ingests a dose meant for a different animal, contact your veterinarian or the ASPCA Animal Poison Control Center immediately (1-888-426-4435 — fee may apply).

📞 ASPCA Animal Poison Control: 1-888-426-4435 (fee may apply)
📞 Pet Poison Helpline: 1-855-764-7661 (fee may apply)
📞 Human accidental ingestion: Poison Control 1-800-222-1222

Secure Storage Required Palatability = Overdose Risk Keep From All Pets & Children Call Poison Control if Ingested

Breaking Development

FDA Granted NexGard Emergency Use Authorization for New World Screwworm

🇺🇸 FDA Emergency Use Authorization • Updated February 2026

🚨 EUA issued for: Treatment of Cochliomyia hominivorax (NWS) larval infestations in dogs in border and travel-risk areas

✅ FDA issued Emergency Use Authorization for NWS treatment in dogs

✅ NWS not detected in the U.S. as of February 18, 2026

⚠️ Risk primarily near U.S.-Mexico border or travel to affected countries

✅ NexGard authorized emergency use for this unapproved indication

📋 Not a standard NexGard use — requires veterinary oversight and EUA protocol

📋 Proper wound care remains essential alongside NexGard treatment

In early 2026, the FDA issued an Emergency Use Authorization (EUA) authorizing NexGard (afoxolaner) for the treatment of New World Screwworm (Cochliomyia hominivorax) larval infestations in dogs, based on its efficacy in killing fly larvae in wounds. As of February 18, 2026, the FDA confirmed that NWS has not been detected in the United States. However, pets near the U.S.-Mexico border and pets that have traveled to countries with active NWS cases face elevated risk. This EUA represents a use of NexGard that is not on the standard approval label — it was granted under emergency authorization specifically for this parasite. Veterinary oversight is required. The NexGard Fact Sheet for Veterinarians issued by the FDA notes: “Proper wound care and management practices are essential for preventing NWS myiasis.” This is a new and evolving development that most pet owners will not need to act on unless they live in or travel to at-risk regions.

🌐 FDA EUA announcement: fda.gov/animal-veterinary/cvm-updates
🌐 NWS status updates: usda.gov/topics/animals/new-world-screwworm
🌐 EUA Fact Sheet: fda.gov/media/191166/download

FDA Emergency Use Auth. NWS Not in U.S. Feb 2026 Border & Travel Risk Areas Requires Veterinary Protocol

Beyond Fleas & Ticks

Vets Prescribe NexGard Off-Label for Mange, Ear Mites — Here’s What the Evidence Shows

🧬 Veterinary Extra-Label Use • Published Clinical Studies Support These Uses

📖 Extra-label uses documented: demodectic mange, sarcoptic mange, ear mites

✅ Demodectic mange (Demodex canis): clinical studies support efficacy

✅ Sarcoptic mange (Sarcoptes scabiei): published clinical evidence

✅ Ear mites: veterinarians prescribe off-label for treatment

📋 Off-label = not on the official FDA label but legally prescribed by vets

⚠️ Dosing and frequency differ from the standard flea/tick protocol

⚠️ Only your vet can determine if off-label use is appropriate

Veterinary Partner (VIN) and PetMD both document that afoxolaner (NexGard) is used extra-label by veterinarians for conditions beyond its FDA-approved flea and tick indications. Clinical studies cited in the PetMD review include Beugnet et al. (2016) in the journal Parasite, which demonstrated efficacy of oral afoxolaner for the treatment of canine generalized demodicosis, and Beugnet et al. on sarcoptic mange. PetMD notes: “Because it contains the active ingredient afoxolaner, NexGard PLUS is also used extra-label to treat demodectic mange, sarcoptic mange, and ear mites in dogs.” Extra-label prescribing is legal in veterinary medicine when a licensed veterinarian determines it is in the best interest of the patient. The dosing schedules and frequencies used for mange treatment differ from the once-monthly flea/tick schedule and require close veterinary supervision to monitor for side effects and treatment response.

📋 Demodectic mange treatment typically requires more frequent dosing — follow vet protocol exactly
📋 Progress monitoring (skin scrapings) is usually part of mange treatment protocol
🌐 VIN Veterinary Partner: veterinarypartner.vin.com (afoxolaner overview)

Off-Label by Vets Demodectic Mange Sarcoptic Mange Ear Mites Different Dosing Protocol

Post-Dose Monitoring Protocol

What to Watch for After Giving NexGard — A Practical Monitoring Guide

👀 Veterinarian-Aligned Protocol • FDA Label Timing • When to Call Your Vet

⏱️ Critical window: First 24–48 hours after administration, especially with first-ever dose

👀 Watch for: vomiting within 2 hours of dosing

👀 Watch for: unusual lethargy or weakness

👀 Watch for: muscle tremors, loss of coordination (ataxia)

🚨 Emergency: any seizure — call vet immediately

✅ Most side effects occur with first dose and resolve on their own

✅ If vomiting within 2 hrs: re-dose with a new chewable per label

The product label advises: “Care should be taken that the dog consumes the complete dose, and treated animals should be observed for a few minutes to ensure that part of the dose is not lost or refused.” Beyond this basic instruction, veterinary experience suggests observing your dog for at least 2 hours after the first dose, and for 24–48 hours across subsequent doses. Signs that warrant a call to your veterinarian include: any seizure or convulsion (call immediately), loss of coordination or stumbling, repeated vomiting, severe lethargy or collapse, hives or facial swelling (signs of allergic reaction), or significant behavioral changes. Most dogs that experience any side effect at all experience only mild, temporary vomiting or diarrhea with the first dose and then tolerate subsequent doses without issue, as confirmed by Veterinary Partner. Maintaining a log of each dose given (date, time, and any observations) helps your veterinarian if a concern arises later.

📞 After-hours emergencies: ASPCA Animal Poison Control 1-888-426-4435
📞 Emergency vet finder: aaha.org/find-a-hospital or pets.webmd.com/find-a-vet
📞 Report to FDA: 1-888-FDA-VETS • fda.gov/reportanimalae

Watch 24–48 Hours Seizure = Emergency Log Every Dose Re-dose If Vomiting Under 2 Hours

Sources: FDA DailyMed NexGard label dailymed.nlm.nih.gov (GABA receptor mechanism; 4 sizes 11.3/28.3/68/136 mg; Tmax 2-12 hr; re-dose if vomiting within 2 hr; stored ≤30°C; no known drug interactions; concomitant meds field study; not a repellent; parasites must bite; prescription only federal law; 8 wk/4 lb minimum; pregnancy not evaluated; secure storage); FDA DailyMed NexGard PLUS (heartworm test required; not effective adult D. immitis; 6 doses after last mosquito exposure; moxidectin mechanism; pyrantel mechanism); FDA FOI Summary NADA 141-406 (>97% D. variabilis; >94% I. scapularis; 100% flea 24 hr 35 days; 81.1% flea at 12 hr day 28; disease risk cannot be excluded per labeling); FDA Fact Sheet isoxazoline fda.gov (neurologic adverse reactions; tremors, ataxia, seizures; dogs without prior history; safe and effective majority); FDA EUA NexGard NWS dogs fda.gov/media/191166/download (February 2026; NWS not detected US Feb 18 2026; border/travel risk; wound care essential); NIH/PMC Drag Tielemans Mitchell 2022 PMC9543253 (MDR1 collies 3.8× max dose; passive diffusion; well tolerated); Drugs.com NexGard adverse events by frequency; Veterinary Partner VIN (vomiting ~4%; diarrhea ~3%; no drug interactions; no MDR1 breed-specific risk; re-dose vomiting under 2 hr rule); PetMD (off-label demodectic sarcoptic mange ear mites; MDR1 no increased risk; Beugnet et al. 2016 Parasite demodicosis); GoodRx DVM-reviewed Aug 2025; Vetster Nov 2025 (MDR1 safe at labeled dose; liver processed; pregnancy not evaluated); Boehringer Ingelheim NexGard PLUS announcement (CAPC year-round broad-spectrum recommendation; heartworm prevalence increasing)

📋 NexGard Dosing at a Glance — Weight-Based Sizing

All doses are once monthly. NexGard is available in four color-coded chewable sizes. Dogs over 121 lbs receive a combination of chewables as directed by their veterinarian. Your veterinarian prescribes the correct size based on your dog’s exact weight. Never split chewables — NexGard is not available in scored tablets.

Dog Weight	Afoxolaner Dose	Chewable Size	Monthly Interval	With/Without Food?
4 – 10 lbs	11.3 mg	Smallest (orange)	Every 30 days	Either
10.1 – 24 lbs	28.3 mg	Small (blue)	Every 30 days	Either
24.1 – 60 lbs	68 mg	Medium (green)	Every 30 days	Either
60.1 – 121 lbs	136 mg	Large (red)	Every 30 days	Either
Over 121 lbs	Combination	Multiple per vet	Every 30 days	Either
Under 8 weeks	Do NOT use	Not approved	N/A	N/A
Under 4 lbs	Do NOT use	Not approved	N/A	N/A

Sources: FDA DailyMed NexGard label dailymed.nlm.nih.gov (4 sizes 11.3/28.3/68/136 mg afoxolaner; color-coded packages; with or without food; min 8 weeks/4 lbs; combination for dogs over 121 lbs per vet). All dosing decisions must be made by a licensed veterinarian.

💸 NexGard by the Numbers — Key Efficacy & Safety Facts

✅ Flea Kill Rate at 8 Hours

>99%

NexGard demonstrated more than 99% effectiveness against adult fleas in a well-controlled laboratory study at 8 hours post-administration. Killing begins as early as 4 hours after the first dose (FDA FOI Summary NADA 141-406).

🥿 Tick Kill at 48 Hours

>97%

More than 97% effectiveness against American dog ticks at 48 hours post-infestation for 30 days in laboratory studies. Black-legged tick effectiveness exceeded 94% at the same timeframe (DailyMed label).

🐕 Tick Species Controlled

5 Species

Black-legged tick, American dog tick, lone star tick, brown dog tick, and longhorned tick. Also prevents Borrelia burgdorferi (Lyme disease) infections by killing the carrier ticks before disease transmission occurs.

⚠️ Most Common Side Effect

~4% Vomiting

Veterinary Partner (VIN) reports vomiting in approximately 4% of dogs and diarrhea in approximately 3% — typically mild and occurring most often with the first dose. Most dogs do not repeat these effects with subsequent doses.

🚨 When to Call Your Vet Immediately After Giving NexGard

Any seizure or convulsive episode — even a brief one. Stop NexGard and do not give another dose without speaking to your veterinarian. Report the event to Boehringer Ingelheim (1-888-637-4251) and the FDA (1-888-FDA-VETS).
Loss of coordination, stumbling, or inability to walk normally (ataxia) — these are neurological symptoms that require immediate veterinary evaluation. Do not wait to see if they resolve on their own.
Facial swelling, hives, or difficulty breathing — signs of an allergic reaction that can escalate quickly and require emergency treatment.
Severe or prolonged vomiting or diarrhea with blood — mild single-episode vomiting is listed as a common side effect, but persistent or bloody gastrointestinal symptoms warrant veterinary attention the same day.

❓ NexGard Questions Answered Plainly

💡 My Dog Has the MDR1 Mutation — Is NexGard Really Safe?

The evidence says yes, at the recommended dose. A 2022 peer-reviewed study published in PMC (Drag, Tielemans, and Mitchell) demonstrated that afoxolaner — the active ingredient in NexGard — is safe in homozygous MDR1-deficient Collie dogs at nearly four times the maximum recommended therapeutic dose. This is reassuring because MDR1 dogs are at higher risk from drugs that rely on P-glycoprotein transport across the blood-brain barrier. Afoxolaner crosses membranes via passive diffusion, bypassing the P-glycoprotein pathway and making MDR1 sensitivity largely irrelevant for this specific drug. PetMD confirms: “Dogs with the MDR1 mutation are not at increased risk for adverse effects.” That said, always disclose your dog’s breed and any known MDR1 status to your veterinarian before starting any new prescription medication, since MDR1 sensitivity matters significantly for other commonly prescribed drugs.

💡 I Found a Tick on My Dog Even Though They Are on NexGard — Did It Fail?

Not necessarily. NexGard is not a repellent — it does not prevent ticks from climbing onto or attaching to your dog. Ticks must begin feeding to absorb afoxolaner. The FDA’s approved label explicitly states: “Parasites need to start feeding on the host to become exposed to afoxolaner.” A tick found on a dog on NexGard that is already dead or dying is evidence the product is working. A living, engorged tick found more than 48 hours after attachment may indicate the tick attached after the 30-day treatment window expired. Always check the date of the last dose. If you find a tick within 48 hours of it attaching, its death is likely imminent. If you find a tick that has been feeding for more than 48 hours and is alive, this warrants a call to your veterinarian to discuss the potential need for tick-borne disease testing. Always remove ticks promptly and correctly — grasp close to the skin, pull upward steadily, and do not twist or crush.

💡 Can I Give NexGard with My Dog’s Heartworm Preventive, or Do I Need NexGard PLUS Instead?

If your dog is already on a separate monthly heartworm preventive (like Heartgard, Interceptor, or Sentinel), you can continue giving both NexGard and your existing heartworm product simultaneously. The field study for NexGard included concomitant use with anthelmintics (including those used for heartworm) with no adverse interactions. NexGard PLUS is a convenience product — it combines fleas, ticks, heartworm, roundworm, and hookworm protection into a single chew. Whether it makes more clinical sense to switch to NexGard PLUS or continue separate products depends on your dog’s full parasite risk profile, medication history, and your veterinarian’s recommendation. If your dog is currently heartworm-positive, you cannot start NexGard PLUS until the dog has been treated with an adulticide — the preventive component only prevents new infections, it does not kill adult heartworms already present.

💡 My Senior Dog Has Kidney or Liver Disease — Is NexGard Safe?

The NexGard label does not list liver or kidney disease as a specific contraindication. However, afoxolaner is metabolized primarily by the liver and eliminated mainly through bile (biliary excretion) with some renal involvement. Vetster notes: “Afoxolaner is processed by the liver, but no specific risks are listed for pets with liver or kidney disease. If your dog has a chronic illness, check with a veterinarian before starting any new medication.” This is genuinely your veterinarian’s call to make based on your dog’s specific blood work, degree of organ impairment, and overall clinical picture. For senior dogs with mild organ changes, NexGard may still be appropriate. For dogs with significant liver or kidney failure, the risk-benefit calculation requires direct veterinary evaluation. Never assume a product is safe for a dog with a chronic illness without explicit veterinarian approval.

💡 Does NexGard Protect Against All Ticks That Cause Disease in the United States?

NexGard’s label covers five tick species: black-legged tick (Ixodes scapularis, primary Lyme carrier), American dog tick (Dermacentor variabilis, Rocky Mountain Spotted Fever carrier), lone star tick (Amblyomma americanum, ehrlichiosis and STARI carrier), brown dog tick (Rhipicephalus sanguineus, ehrlichiosis and RMSF carrier), and longhorned tick (Haemaphysalis longicornis, a relatively newer invasive species in the U.S.). Veterinary Partner notes that afoxolaner has demonstrated efficacy against additional tick species not on the official label, though these are not part of the formal approval. The most important limitation to remember: NexGard kills ticks after they begin feeding — it is not a contact repellent. For Lyme disease prevention specifically, the killing window (within 24–48 hours of biting) is shorter than the typical 36–48-hour transmission time for Lyme, making it an effective preventive tool in most exposures. However, no product eliminates all disease transmission risk to zero.

💡 Should I Give NexGard with Food or Without?

NexGard can be given with or without food, per the FDA-approved label. Giving it with food may reduce the chance of vomiting in dogs with sensitive stomachs. The pharmacokinetics show that afoxolaner and moxidectin (in NexGard PLUS) are actually absorbed more rapidly in the fasted state, reaching peak concentration slightly earlier, but the overall area under the curve — meaning total drug exposure over time — is similar whether given with food or without. For practical purposes, giving NexGard as a treat with or after mealtime is perfectly appropriate and may improve palatability acceptance in dogs that are pickier. The key instruction from the label: observe your dog for a few minutes after giving the chew to confirm the complete dose was consumed and not dropped or partially eaten. If part of the dose is lost, contact your vet about whether re-dosing is appropriate.

Sources: NIH/PMC Drag Tielemans Mitchell 2022 PMC9543253 (MDR1 3.8× max dose; passive diffusion; well tolerated; no serious adverse effects); PetMD (MDR1 no increased risk); FDA DailyMed NexGard label (parasites must bite to be exposed; not a repellent; with or without food; pharmacokinetics fed/fasted; concomitant anthelmintic field study); Vetster Nov 2025 (liver processed; no specific risk listed for liver/kidney disease; check with vet for chronic illness); Veterinary Partner VIN (extra tick species off-label efficacy; biliary excretion; renal elimination; no known drug interactions); DailyMed NexGard PLUS (not effective adult D. immitis; fasted Tmax 2-3 hr; fed Tmax slightly longer; AUC similar fed/fasted); FDA FOI NADA 141-406 (disease risk from parasites not excluded per labeling)

✅ Five Things to Discuss With Your Vet Before Starting NexGard

Your dog’s complete seizure and neurological history. Even a single episode of unexplained tremors, fainting, or strange behavior in the past is worth disclosing. The FDA has formally flagged neurological risks for isoxazoline products including NexGard, and your vet needs full history to make the right call for your specific dog.
Your dog’s breed and any known MDR1 genetic status. If your dog is a Collie, Shetland Sheepdog, Australian Shepherd, or another herding breed, mention it. While research shows NexGard is safe in MDR1-affected dogs, the disclosure ensures your vet has the full picture for all prescriptions.
Whether your dog may be pregnant, nursing, or used for breeding. The safety of NexGard has not been evaluated in these populations. Your vet may recommend alternative parasite control protocols during these periods.
The heartworm status of your dog if you are considering NexGard PLUS. A heartworm antigen test is required before starting any heartworm preventive, including NexGard PLUS. Do not skip this step — giving a preventive to a heartworm-positive dog carries real clinical risk.
Every other medication, supplement, or treatment your dog currently receives. While no drug interactions with afoxolaner are currently documented, full disclosure allows your vet to monitor appropriately and update their guidance if new interactions are identified in the future.

⚠️ Three Mistakes Pet Owners Make With NexGard

Assuming NexGard keeps ticks away — it does not. NexGard is not a repellent. Ticks will still attach to your dog and begin feeding. They die within hours to days of biting, but they must bite first. Tick checks after outdoor time remain important even when your dog is on NexGard.
Skipping doses “in winter” because fleas and ticks seem less active. The Companion Animal Parasite Council recommends year-round monthly prevention because fleas survive indoors year-round and ticks can be active whenever temperatures are above freezing. Many tick-borne disease exposures happen in late fall and early spring when people let their guard down.
Purchasing NexGard without a prescription from unofficial online sources. NexGard is a federally restricted prescription drug. Purchasing it from any source that does not require a valid veterinary prescription means receiving a product outside legal channels that may be counterfeit, improperly stored, or expired. Only buy from licensed veterinary pharmacies with a valid Rx.

© BestiePaws.com — This guide is independently researched and written for educational purposes only. We are not affiliated with, compensated by, or endorsed by Boehringer Ingelheim Animal Health, the FDA, or any veterinary organization. NexGard is a registered trademark of Boehringer Ingelheim Animal Health France, used under license. All drug information is sourced from official FDA labeling, peer-reviewed research, and clinically reviewed veterinary references as of March 2026. Veterinary drug information changes — always confirm current prescribing information with your licensed veterinarian. This content is not veterinary advice and does not replace a professional veterinary examination. 📞 Report adverse events: 1-888-FDA-VETS • Boehringer Ingelheim: 1-888-637-4251 • ASPCA Poison Control: 1-888-426-4435 • Emergency vet: aaha.org/find-a-hospital

Primary sources: FDA DailyMed NexGard afoxolaner label dailymed.nlm.nih.gov/drugInfo.cfm?setid=a668d223 (full prescribing information; efficacy data; adverse events; storage; dosing; concomitant meds; approved tick species; Lyme disease prevention; puppies 8 wk/4 lb minimum; pregnancy not evaluated); FDA DailyMed NexGard PLUS label dailymed.nlm.nih.gov/drugInfo.cfm?setid=1eb7bc46 (afoxolaner + moxidectin + pyrantel; heartworm test before starting; not effective adult D. immitis; 6 months after last mosquito exposure; MDR1 safety; pharmacokinetics fed/fasted); FDA FOI Summary NADA 141-406 animaldrugsatfda.fda.gov (flea kill 4 hr start; >99% at 8 hr; 100% flea 24 hr; >97% D. variabilis; >94% I. scapularis; >93% R. sanguineus; >97% A. americanum; Lyme prevention studies; margin of safety 8-9 wk puppies 1×/3×/5× dose); FDA Fact Sheet isoxazoline products fda.gov/animal-veterinary (neurologic adverse reactions; tremors ataxia seizures; dogs without prior history; safe effective majority; petowners may report); FDA EUA NexGard NWS dogs fda.gov/media/191166/download rev 02/2026 (NWS dogs EUA; February 18 2026 NWS not in US; border travel risk; wound care); NIH/PMC Drag Tielemans Mitchell 2022 PMC9543253 (MDR1 collie safety 3.8× max dose; passive diffusion mechanism; no serious adverse effects); Drugs.com NexGard adverse events frequency ranking (updated 2026-03-01); Veterinary Partner VIN veterinarypartner.vin.com (vomiting ~4%; diarrhea ~3%; re-dose under 2 hr; no drug interactions; biliary excretion; extra tick species); PetMD petmd.com (off-label demodectic sarcoptic mange ear mites; MDR1 no increased risk; Beugnet 2016 Parasite; esafoxolaner for cats separate); GoodRx DVM-reviewed Aug 2025 (Sarah Wooten DVM CVJ); Vetster Nov 2025 (liver processing; MDR1 safe at labeled dose; pregnancy not evaluated; seizure risk)

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