How Much Benadryl is Fatal for a Cat?

Benadryl is a widely used over-the-counter medication that provides relief from the symptoms of allergies and other conditions. Unfortunately, it can be fatal for cats if they ingest it. Always read the label and follow the directions carefully when administering this medication.

Lethal Dose of Benadryl for Cats

Can a cat overdose on Benadryl?

Benadryl is an antihistamine that can be used to treat allergic reactions and relieve the symptoms of allergies in cats. It can also be used for motion sickness and as a sedative for short-term management of insomnia in cats.

However, giving your cat too much Benadryl may cause:

  • Drowsiness
  • Lethargy
  • Vomiting
  • Agitation or excitability
  • Dizziness or disorientation
  • Diarrhea
  • Increased heart rate or blood pressure
  • Decreased body temperature

Rarely, an overdose can result in life-threatening cardiac arrhythmias or seizures.

The lethal dose of Benadryl for cats

Some cats may react negatively to the side effects of this drug at any dose; others may experience severe side effects from taking too much Benadryl. The lethal dose of oral Benadryl in cats may be achieved with approximately 20 – 40 tablets. With pre-existing diseases, a lower dose of Benadryl may have been sufficient to cause death.

The lethal dose of Benadryl in cats has not been defined and is variable. However, the lowest reported oral dose that caused death was about 7 tablets for a 10-pound cat (4.5 kg).

Therefore, it is reasonable to recommend that you do not give your cat more than 2-3 tablets at one time. It is also recommended that you never give your cat Benadryl without first discussing dosing with your veterinarian to avoid accidental overdose and the need for veterinary care.

What are the side effects of Benadryl?

Before you give your cat Benadryl, it’s important to understand the side effects this medication can cause. Some of the more common side effects are:

  • Drowsiness
  • Lethargy
  • Disorientation
  • Excessive thirst
  • Dilated pupils
  • Urinary retention (inability to urinate)

In general, cats tolerate Benadryl well when given at the proper dose. Cats with underlying medical conditions are more likely to experience side effects.

How much Benadryl can a cat have?

For cats, the standard dose of Benadryl is 1 mg/lb every 8 to 12 hours. So, if your cat weighs 10 pounds, you would give him 10 mg two to three times a day.

Your veterinarian may suggest a different dosage for your particular cat, so check with them before administering any medication.

If you are giving your cat children’s Benadryl chewable tablets, make sure they are unflavored, as the flavor contains xylitol which is poisonous to cats.

How long does it take Benadryl to kick in for a cat?

The answer depends on many factors, including the reason that you are giving your cat Benadryl. However, if the medicine is given orally, it should start working within 30 minutes.

This medication is available in both tablet and liquid form, so if your cat won’t take the tablet, you can try liquid Benadryl.

Why is my cat foaming at the mouth after taking Benadryl?

If you give your cats Benadryl, they may foam at the mouth because they don’t like the taste. Sometimes foaming at the mouth can be a sign of an allergic reaction to Benadryl; however, allergic reactions in cats are extremely rare.

If you’re concerned about your pet’s health after an episode of foaming at the mouth, contact your veterinarian immediately.

What is the best way to give your cat Benadryl?

You can give your cat Benadryl in a variety of ways, but the easiest and most effective way to dose a cat is to give him liquid Benadryl in a syringe. A syringe will help you get the medicine into your cat without fuss or muss.

Can you put a cat to sleep with Benadryl?

No, you can’t put a cat to sleep with Benadryl. While small doses of Benadryl (1-2 mg per pound) can be given to cats for allergies and motion sickness, large doses are toxic and can cause central nervous system depression. In rare cases, this could cause an overdose leading to liver failure, coma, and death.

If your cat is ill or experiencing distress, it’s important to seek medical care from a veterinarian who is familiar with your pet’s medical history. They can determine the underlying cause of the issue and administer appropriate treatment or medication.

As pet owners, we know our cats may be uncomfortable or in pain when they’re ill. You may be tempted to give your cat Benadryl if you believe it will ease their discomfort and make them more comfortable until you can get help from a veterinarian. However, it’s not safe to use this medication as a sedative on your cat.

Note: Benadryl does not work as a pet euthanasia agent because it does not cause rapid unconsciousness. Pets given Benadryl can become very sleepy, but they will typically wake up within a few hours and will likely experience discomfort when the drug wears off.


Hannah Elizabeth is an English animal behavior author, having written for several online publications. With a degree in Animal Behaviour and over a decade of practical animal husbandry experience, Hannah's articles cover everything from pet care to wildlife conservation. When she isn't creating content for blog posts, Hannah enjoys long walks with her Rottweiler cross Senna, reading fantasy novels and breeding aquarium shrimp.

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