Amoxicillin is one of the most frequently prescribed antibiotics in veterinary medicine — and also one of the most frequently misunderstood by cat owners. The FDA-approved label says once a day. Current veterinary practice says that’s not enough. The tablet dose cap changes for larger cats. And giving it the wrong way, or stopping early, can make the infection worse. This guide covers every piece of that picture, with an interactive dosage calculator and a complete reference chart.
mg/kg Current veterinary practice dosage range — every 8–12 hours (not the once-daily FDA label)
🧮 Amoxicillin Dosage Calculator for Cats
Enter your cat’s weight and select the dosing level your vet has indicated. This produces an estimated reference range — your vet’s prescription is the only dose your cat should actually receive. Do not use this to self-prescribe.
The questions cat owners search in the middle of the night when the prescription isn’t making sense or when something feels off. Answered plainly, with the same information a vet would give in the exam room.
This is the most common point of confusion about amoxicillin for cats, and it comes from a genuine gap between the FDA-approved label and current veterinary practice. The FDA-approved labeled dose for cats is 50 mg once daily (approximately 11–22 mg/kg once a day). However, most veterinary textbooks and current small animal medicine guidelines no longer consider once-daily dosing sufficient for most infections. Amoxicillin is a time-dependent antibiotic — it works by maintaining therapeutic levels in the bloodstream above the bacterial minimum inhibitory concentration, and cats metabolize it quickly enough that once-daily dosing creates long periods below that level. Most vets now prescribe it every 8–12 hours — the same interval used for dogs — and that is considered the current standard of care for most feline infections. If your prescription says twice daily or three times daily, that is correct and based on current evidence, not a dispensing error.
This is one of the less-intuitive aspects of amoxicillin dosing in cats. At the standard rate of 10 mg/kg, a 7.7-pound (3.5 kg) cat would receive approximately 35 mg per dose. A 15-pound (6.8 kg) cat would calculate to 68 mg — but most veterinarians cap the per-dose amount at 50 mg for cats. The reason: clinical evidence and safety data both suggest that doses above 50 mg per administration in cats increase the risk of adverse effects without proportionally improving efficacy. This means a 7-pound cat and a 13-pound cat may end up receiving the same 50 mg tablet, even though the math suggests otherwise. Your vet is applying this cap intentionally, not making an error. If your large cat’s prescription looks lower than expected relative to weight, this is almost certainly why.
Yes — and this matters more with antibiotics than with most other medications. When a cat appears to improve, the most susceptible bacteria have been killed and the most drug-resistant ones remain. Stopping early allows those resistant bacteria to regrow and multiply — and now they’re more resistant to amoxicillin than the original population was. The next round of treatment may require a stronger or more expensive antibiotic, or may fail entirely. Course lengths are infection-specific: skin abscesses and simple infections typically run 5–7 days or until 48 hours after all visible signs resolve; UTIs require 10–14 days because the urinary tract maintains higher drug concentrations and the bacteria are embedded in the bladder wall; more complex infections may run longer. The course length on the prescription is the minimum necessary to achieve bacterial clearance — not an estimate of when your cat will feel better.
No — and this has several layers to it. First and most importantly, it is illegal in the United States to give a cat a prescription human medication without a current veterinary prescription for that specific cat. Second, human amoxicillin capsules contain 250 mg or 500 mg — far above the 50 mg per-dose cap for cats — and accurate small-animal dosing from these capsules requires dividing them into fractions that almost no owner can do accurately enough. Third, some human liquid formulations contain xylitol, artificial sweeteners, or other inactive ingredients that are toxic to cats. The same applies to fish amoxicillin (sold without a prescription as aquarium treatment): even if the active ingredient is chemically identical, you have no way to verify purity, accurate dosing, or absence of harmful inactive ingredients. The risks of using either are real and the cost savings over a proper veterinary prescription are minimal.
Clavamox is amoxicillin combined with clavulanic acid — and the difference matters clinically, not just commercially. Clavulanic acid inhibits beta-lactamase, an enzyme that some bacteria produce specifically to break down penicillin antibiotics and render them ineffective. By blocking this enzyme, clavulanic acid allows the amoxicillin to work against bacteria that would otherwise be resistant to it. The FDA-approved dose for Clavamox in cats is 62.5 mg (1 mL) twice daily. Clavamox is typically prescribed when your vet suspects a bacteria that produces beta-lactamase — common in bite wound infections, some skin infections, and certain UTI bacteria — or when a plain amoxicillin course has already failed. It’s not simply a stronger version; it’s a combination designed to defeat a specific resistance mechanism. It is generally more expensive than plain amoxicillin and is not interchangeable with it.
Probably not. Once the dry powder (Amoxi-Drop) is mixed with water to make the liquid suspension, it must be discarded after 14 days — even if doses remain. This is not a conservative suggestion; it’s the manufacturer’s specification based on potency testing. Refrigeration is preferred for the reconstituted suspension but is not strictly required by the label. Tablets (Amoxi-Tabs) can be stored at room temperature between 59°F and 86°F in a dry, dark place and have a longer shelf life. Never freeze either form. The odd smell of reconstituted amoxicillin suspension is normal — it has a distinctive fruity/medicinal smell that’s part of the formulation, not a sign of spoilage. The sign to actually watch for is a change in appearance or consistency compared to when it was first reconstituted. When in doubt, contact your vet — an expired antibiotic that appears to treat but doesn’t reach therapeutic levels is one of the causes of antibiotic resistance development.
Yes, but with an important distinction. The majority of feline upper respiratory infections (URIs) are caused by viruses — herpesvirus and calicivirus together account for the bulk of cat URI cases — and amoxicillin has no effect on viruses. However, secondary bacterial infections frequently complicate viral URIs, causing the yellow or green discharge, worsening congestion, and fever that develop a few days after the initial viral symptoms. In those secondary bacterial infections, amoxicillin is appropriate and effective — it targets common opportunistic bacteria like Staphylococcus, Streptococcus, and some strains of Pasteurella that colonize the already-inflamed respiratory tract. Your vet may prescribe amoxicillin for a URI if they see evidence of bacterial secondary infection — not because amoxicillin treats the underlying virus, but because the bacterial component is treatable and removing it allows the cat to recover faster from the viral infection.
Don’t re-dose without calling your vet. This is a situation where the timing matters: if the vomiting happened within about 15–20 minutes of the dose — before absorption — your vet may advise giving a replacement dose, possibly with a small amount of food to settle the stomach. If vomiting occurred later, enough of the dose may have been absorbed that a replacement would risk over-dosing. The cardinal rule: do not double up on any dose, even a missed one. Missing a single dose does not require doubling the next one — just resume the regular schedule at the next scheduled time. For persistent vomiting after every dose, your vet can discuss options including giving the medication with food, switching to a different antibiotic that causes less GI upset, or adjusting the dose timing. A cat that consistently refuses or vomits liquid amoxicillin may accept it better at room temperature rather than cold from the refrigerator — the smell and texture are less abrasive at room temperature.
The chart below reflects current veterinary practice dosing at 10 mg/kg with the 50 mg per-dose cap applied for cats over approximately 7.7 lbs (3.5 kg). Your vet may prescribe a different amount based on the specific infection, the cat’s health status, and the interval selected. Always follow your vet’s prescription above any chart.
| Weight (lbs) | Weight (kg) | Standard Dose (10 mg/kg) | Typical Tablet/Liquid | Frequency | Note |
|---|---|---|---|---|---|
| 2–4 lbs | 0.9–1.8 kg | 9–18 mg | Liquid suspension only | Q12h (every 12 hrs) | Kittens: vet determines; extra caution in very young |
| 5 lbs | 2.3 kg | ~23 mg | Liquid suspension | Q12h | Liquid preferred for accurate small doses |
| 6 lbs | 2.7 kg | ~27 mg | Liquid suspension | Q12h | Liquid preferred for accurate small doses |
| 7 lbs | 3.2 kg | ~32 mg | Liquid or ½ tablet | Q12h | Approaching 50 mg cap range |
| 8–10 lbs | 3.6–4.5 kg | 50 mg (capped) | 50 mg tablet (Amoxi-Tab) | Q12h | 50 mg cap applies — same dose as heavier cats |
| 11–13 lbs | 5.0–5.9 kg | 50 mg (capped) | 50 mg tablet (Amoxi-Tab) | Q12h or Q8h (vet-directed) | Weight-based math exceeds cap — 50 mg held |
| 14–16 lbs | 6.4–7.3 kg | 50 mg (capped) | 50 mg tablet | Q8–12h (vet-directed) | Higher frequency may compensate for large size |
| Over 16 lbs | Over 7.3 kg | 50 mg (capped) | 50 mg tablet | Vet-directed only | Vet may adjust frequency rather than dose amount |
- Urinary tract infection (UTI / cystitis): 11–15 mg/kg every 8–12 hours · 10–14 days · longer for pyelonephritis
- Upper respiratory tract infection (secondary bacterial): 11–22 mg/kg every 8–12 hours · 7–10 days
- Skin and soft tissue infections / abscesses: 11–22 mg/kg every 8–12 hours · 5–7 days, or 48 hours after all signs clear
- Gastrointestinal tract bacterial infections: 11–22 mg/kg every 8–12 hours · 5–7 days
- Dental infections (pre- or post-procedure): Vet-directed; often 5–7 days following dental procedure
Amoxicillin is not a cure-all antibiotic. It works against specific types of bacteria through a specific mechanism. Knowing what it covers — and crucially, what it doesn’t — is information most owners never get at the pharmacy window.
Secondary bacterial upper respiratory infections (not the viral URI itself, but the bacterial complication). Urinary tract infections involving susceptible bacteria. Skin infections, puncture wounds, and abscesses — particularly bite wounds from other cats. Dental infections and post-surgical dental care. Some gastrointestinal bacterial infections. Soft tissue infections. According to the Merck Veterinary Manual, amoxicillin has specifically strong performance in feline UTI treatment, rising to therapeutic levels in the urinary tract rapidly after an oral dose. A study of 122 cats with infected skin wounds or abscesses found a 95.1% bacterial clearance rate with amoxicillin treatment.
When a culture and sensitivity test shows the bacteria is resistant. When the infection is caused by Mycoplasma (requires doxycycline or azithromycin). When a previous amoxicillin course failed for the same infection — this often signals beta-lactamase producing bacteria, in which case Clavamox is needed. When the cat has a known penicillin allergy. When the cat is in septic shock or cannot absorb oral medications — IV antibiotics are required in these situations and oral amoxicillin will not reach therapeutic levels. Amoxicillin is also not suitable for most Bordetella infections, and its activity against some Staphylococcus strains has been reduced by resistance patterns documented over the last decade.
Medicating a cat is one of the more genuinely difficult parts of pet ownership. The approach that works depends on the cat’s personality and the form of the medication. What follows is the practical reality — not the idealized instructions on the box.
- Amoxi-Drop (liquid suspension): The most common form dispensed for cats. Must be shaken well before each dose. Preferred for small cats where accurate dosing of fractions of a tablet would be difficult. Store liquid suspension in the refrigerator (preferred, not required). Discard any remaining liquid after 14 days regardless of how much remains.
- Amoxi-Tabs (tablets): Come in 50 mg, 100 mg, 150 mg, 200 mg, and 400 mg sizes. Store at room temperature between 59–86°F in a dry, dark place. Longer shelf life than reconstituted liquid. Never crush and mix with a large amount of food — the cat may eat only part of the meal and receive only part of the dose.
- Compounded formulations: Some pharmacies prepare amoxicillin in transdermal gels or flavored suspensions for cats that refuse oral medication. These require a veterinary prescription and are not interchangeable with standard formulations in terms of dose and absorption.
Warm the syringe in your hand for a minute before approaching if the suspension has been refrigerated — cats are much more likely to accept it at room temperature. Hold the cat on your lap facing away from you, or wrap loosely in a towel (the “burrito method”) to limit sudden movement. Insert the syringe at the side of the mouth behind the canine tooth — not straight in, which triggers a gag reflex. Deposit the liquid slowly in small pulses against the cheek. If the cat spits out some liquid, do not re-dose. Simply resume the original dosing schedule at the next scheduled time. Giving more risks overdose, and a small amount of lost medication is not clinically meaningful for a short-term antibiotic. Never mix the liquid into a large bowl of water — you cannot guarantee the cat will drink all of it.
Pill pockets, a small piece of deli turkey, a soft treat, or a tiny ball of cream cheese can conceal a tablet for cats that accept treats willingly. The trick: give an empty treat first, then the treat with the tablet, then immediately another empty treat — the second empty treat encourages swallowing the pill-containing one and distracts the cat from spitting it out. If your cat discovers the pill and spits it out, a pill gun (pill popper) is the next step. After giving a tablet without food, offer a small syringe of water (about 1–2 mL) by mouth to make sure the tablet moves into the stomach — dry-pilling cats risks esophageal lodging of the tablet, which can cause esophageal damage and is more common in cats than in dogs because cats have less effective swallowing reflexes for solid objects without a liquid follow-up.
Amoxicillin can be given with or without food. If GI upset occurs after a dose, giving it with a small meal reduces that effect without meaningfully changing absorption. For a missed dose: give it as soon as you remember — unless the next dose is due within a couple of hours, in which case skip the missed dose and resume the regular schedule. Never give two doses at once to make up for a missed one. For twice-daily dosing, space doses approximately 12 hours apart; for three-times-daily, space them approximately 8 hours apart. Try to keep the schedule consistent. An alarm on your phone set for dose times is the single most effective compliance tool for multi-day antibiotic courses — irregular dosing is one of the most common causes of treatment failure.
Amoxicillin is one of the safest antibiotics used in veterinary medicine — serious problems at prescribed doses are uncommon. But knowing the difference between a manageable side effect and a veterinary emergency can save a cat’s life.
A single slightly excess dose is unlikely to cause serious harm in a cat that tolerates amoxicillin normally — GI upset (vomiting, diarrhea) is the typical response. Large overdoses are a different situation: extremely high doses can cause neurological signs including incoordination and seizures, kidney inflammation and damage, crystalluria (crystals in the urine causing urinary straining or blood in urine), and in severe cases, kidney failure. If you suspect an overdose — your cat got into the bottle, received another pet’s dose, or you gave a dose in error — call your veterinarian immediately, or call ASPCA Poison Control at 888-426-4435 (consultation fee applies). Do not wait for symptoms to appear. Symptoms of significant overdose can take hours to become visible, and early intervention produces far better outcomes than waiting until the cat is showing obvious signs.
Amoxicillin has a relatively short list of significant interactions compared to some other antibiotics, but several matter. Tetracycline antibiotics (doxycycline): combining amoxicillin with a tetracycline reduces amoxicillin’s effectiveness — bacteriostatic antibiotics like tetracyclines slow bacterial growth, which reduces the activity of bactericidal antibiotics like amoxicillin that need actively growing bacteria to work. Mycophenolate: amoxicillin reduces the absorption and effectiveness of mycophenolate, a medication used for some immune-mediated conditions in cats. Urine glucose tests: amoxicillin can cause false-positive results on certain urine glucose dipstick tests — tell your vet if your cat is being monitored for diabetes. Other antibiotics generally: avoid combining amoxicillin with macrolides, sulfa drugs, or other antibiotics unless your vet has specifically directed the combination.
- Cats with a known penicillin or cephalosporin allergy — cross-reactivity exists between penicillin-class antibiotics and some cephalosporins. Drug sensitivities can also develop with repeated exposure, so always watch for signs throughout the full course, not just at the first dose.
- Cats in septic shock or severe systemic illness — oral medications cannot be reliably absorbed in cats with poor circulation. IV antibiotics are required.
- Cats with severe kidney disease — amoxicillin is primarily excreted by the kidneys; dose adjustment may be needed and your vet should be made aware of any kidney disease diagnosis before prescribing.
- Cats on certain immunosuppressive medications — particularly mycophenolate; always disclose all current medications to your vet before a new prescription is issued.
Stop giving the medication immediately. Signs of allergic reaction include facial swelling, hives or skin rash, excessive scratching or pawing at the face, difficulty breathing, and collapse. A mild reaction — mild hives that resolve quickly — still requires a call to your vet before the next dose; they may switch to a different antibiotic class. A severe reaction with breathing difficulty or collapse is a veterinary emergency — go to the nearest open emergency clinic right now, and call ahead if possible so they can prepare. True penicillin allergy in cats is rare but it does exist, and once a reaction occurs, the cat should never receive amoxicillin, ampicillin, or any penicillin-class antibiotic again. Make sure this is documented in the cat’s records at every future visit.
You have more options than you might think. First, try serving it at room temperature rather than cold from the refrigerator — the smell and texture are more palatable at room temperature, and cold liquid triggers more resistance in most cats. Second, try the syringe at the corner of the mouth rather than the front — placing it behind the upper canine tooth and angling toward the cheek pouch causes much less gagging. Third, ask your vet or pharmacist whether a compounded flavored version (chicken, fish, or beef flavor, sometimes in a transdermal gel applied to the ear flap) would be appropriate — this is an actual option and some cats who refuse oral liquid accept a gel wiped on the inside of the ear. Do not attempt to mix the liquid in a full bowl of food — you can never guarantee the cat eats all of it and therefore received the full dose.
No — and this matters more than most owners realize. Complete the full prescribed course. When a cat appears better early in a course, the bacteria that were most vulnerable to amoxicillin have been killed — but the more resistant ones may still be present in smaller numbers. Stopping early gives those survivors the chance to multiply and establish a harder-to-treat infection. The next course of antibiotics for the same infection may need to be stronger, longer, or more expensive — and may not clear the infection as completely. This is one of the primary drivers of antibiotic resistance in veterinary medicine. If you’re concerned the medication is causing problems, call your vet — stopping early without guidance is almost never the right answer, but your vet may be able to switch to a different formulation that’s easier to tolerate for the remaining days.
This is valuable information and your vet needs to know it explicitly. There are two scenarios: the infection wasn’t bacterial in the first place (which antibiotics never help), or the bacteria has developed or already had resistance to amoxicillin. If the latter, Clavamox (amoxicillin-clavulanate) is often the next step — the clavulanate component blocks the enzyme that makes bacteria resistant to plain amoxicillin. For recurrent infections or when amoxicillin has failed twice, your vet may recommend a culture and sensitivity test, which identifies exactly which bacteria is causing the infection and which antibiotics it’s susceptible to. This test eliminates guesswork and is particularly valuable for chronic UTIs, recurrent skin infections, and persistent respiratory problems. Never assume a previous antibiotic failure means the infection is untreatable — it usually means a different antibiotic is needed.
This requires a direct conversation with your vet before the prescription is filled. Amoxicillin is primarily excreted through the kidneys, and cats with reduced kidney function may clear it more slowly — meaning drug levels remain elevated in the body for longer than intended. This is relevant both for therapeutic window (too little is ineffective, too much risks toxicity) and for the kidneys themselves, since extremely high doses carry kidney damage risk. Your vet may reduce the dose, extend the interval between doses, or choose a different antibiotic with a different excretion pathway. Do not assume a standard prescription is appropriate for a cat with documented CKD (chronic kidney disease). If a prescription is written without the vet knowing about the kidney diagnosis, tell them before you start the medication.
Call your vet. A lack of response after 48–72 hours is a meaningful clinical signal, not just normal variation. The most common explanations: the infection is viral (antibiotics have no effect), the bacteria is resistant to amoxicillin, the diagnosis is incomplete (an abscess that needs draining in addition to antibiotics, or a foreign body causing the infection), or the cat is not absorbing the medication due to concurrent illness. Do not simply add more doses, increase the amount, or try a different human antibiotic from your medicine cabinet. The right next step is a recheck with your vet — who may want to run a culture, drain an abscess, change the antibiotic, or investigate whether the original diagnosis was correct. Persistent infections that don’t respond to first-line antibiotics are routinely treatable once the right drug is identified.
This guide and calculator are for general educational reference only and do not constitute veterinary medical advice, diagnosis, or treatment. Amoxicillin is a prescription-only antibiotic in the United States — it cannot legally be given to a cat without a current prescription from a licensed veterinarian for that specific animal. Dosage information reflects current veterinary literature and reference guides including Plumb’s Veterinary Drug Handbook and current small animal medicine guidelines, but individual dosing for your cat will be determined by your veterinarian based on your cat’s specific diagnosis, weight, age, kidney and liver function, concurrent medications, and clinical judgment. Never give a cat human amoxicillin, fish amoxicillin, or any antibiotic without a valid veterinary prescription. If your cat is experiencing a medical emergency or potential overdose, call your veterinarian immediately, contact ASPCA Poison Control at 888-426-4435 (fee applies, 24/7), or go to the nearest emergency veterinary hospital. This content is entirely original.