Petco Text Logo
Petco Pet Logo

  • Two Easy Ways to Start Earning Rewards!

Benadryl for Dogs & Cats: Uses, Dosage, Side Effects, & Safety

Published on September 10th, 2024
Updated on October 15th, 2025

Antihistamine used primarily in veterinary medicine for treating the symptoms associated with allergic disease and allergic reactions; variably effective. Use in veterinary medicine is extra label. 

Benadryl®

Summary

Used For:

Allergies, acute allergic reactions, motion sickness, mild sedation, some anti-nausea effects

Brands:

Benadryl®

Species:

Dogs, cats, ferrets, guinea pigs, rabbits, birds (Psitticines), horses, cattle

Other names:

iphenhydramine, Allermax, Banophen, Compoz Nighttime sleep aid and many others

Drug type:

Antihistamine

Prescription required:

No

Pregnancy and lactation:

Safe if not administered when the animal is near term

Side effects:

Lethargy, sleepiness, dry mouth, urine retention, mild GI signs possible
 

General description

Diphenhydramine is the generic name for an antihistamine that has many uses in veterinary medicine. This drug is typically given as an injection to animals that are experiencing a reaction to vaccinations or blood transfusion, or other causes of anaphylaxis. The human drug Benadryl® is available over-the-counter in most pharmacies, and does have some efficacy for treating mild allergic disease in veterinary patients, however efficacy is often highly variable from patient to patient, and the liquid form of the medication is very difficult to administer to veterinary patients due to the taste. It is strongly recommended that any medication be used in consultation with your veterinarian.

Uses

Reducing itchiness associated with allergic skin disease, treating severe allergic reactions associated with blood product transfusions or vaccine reactions, pretreatment before vaccination in known reactors

How it works

Diphenhydramine prevents the binding of compounds called histamines to the body’s histamine receptors. Histamines are produced in the body and are released during periods of acute and chronic inflammation.

Dosage

Benadryl use in pets is extra-label. Please consult your veterinarian before you give Benadryl to your pet. **Dogs:** For allergies: 2 – 4 milligrams per kilogram of body weight by mouth every 8 – 12 hours; for allergic reactions or anaphylaxis: 0.5 – 2 milligrams per kilogram of body weight subcutaneously, intramuscularly, or intravenously (extra-label); for prevention of motion-sickness or for reduction of nausea: 2- 4 milligrams per kilogram of body weight by mouth or intramuscularly every 8 hours **Cats:** For allergies: 2 – 4 milligrams per kilogram of body weight by mouth every 8 – 12 hours; for allergic reactions or anaphylaxis: 0.5 – 2 milligrams per kilogram of body weight subcutaneously, intramuscularly, or intravenously (extra-label) **Ferrets:** To prevent vaccine reactions: 2 milligrams per kilogram of body weight by mouth, intravenously, or intramuscularly 10 minutes prior to vaccination; to prevent reactions to the chemotherapeutic drug doxorubicin: 5 milligrams intramuscularly **Guinea pigs:** 2 to 5 milligrams per kilogram of body weight by mouth **Rabbits:** 1 – 2 milligrams per kilogram of body weight by mouth every 12 hours **Birds:** To help treat itchiness caused by feather picking in parrots: 2 milligrams per kilogram of body weight by mouth every 12 hours **Cattle:** To help treat anaphylaxis: 0.5 – 1 milligram per kilogram of body weight intramuscularly or intravenously; to help treat aseptic laminitis: 55 – 110 milligrams per 100 kilograms of body weight intravenously or intramuscularly

Forms

Diphenhydramine HCl capsules and tablets in 12.5 milligram, 25 milligram, and 50 milligram sizes; Diphenhydramine oral liquid in 12.5 milligrams per 5 milliliters and 25 milligrams per 5 milliliters strengths; Diphenhydramine solution for injection, 50 milligrams per milliliter

Storage information & handling

Store at room temperature, do not freeze.

What results can I expect?

Reducing in itchiness and redness of skin in the case of allergic disease. For severe allergic reactions and anaphylaxis, produces reduction of swelling and resolution of other signs, especially when used in conjunction with corticosteroids. Typically produces drowsiness, although animals become refractory to this effect over time.

Missed Dosage

If a dose is missed, continue with the next dosage at the scheduled time.

Overdose / acute toxicity

Overdose is uncommon but can produce excitement, seizures, depression (lethargy to coma), dryness of the mouth, respiratory depression and death.

Side effects

* Lethargy * Sleepiness * Dryness of the mouth * Retention of urine * GI effects (vomiting, diarrhea, decreased to absent appetite)

Contraindications

* Previous sensitivity * Angle closure glaucoma * Enlarged prostate * Obstruction of the pylorus or bladder neck * COPD * Hyperthyroidism * Cardiac Disease * High blood pressure

Drug interactions

The actions of some drugs in the anticholinergic class (i.e. atropine) may be potentiated when given with Benadryl®. Increased sedation may occur with drugs that depress the central nervous system.

Precaution

In most healthy animals Benadryl® is a safe drug to use, however discuss any potential contraindications your pet may have with your veterinarian before using.

Disclaimer
The information contained on this page is for educational purposes only. This medication should only be given under the advice of a veterinarian who has examined your pet under the laws applicable to your state of residence.

Related Questions

Petco Pet Education Center
Health & Wellbeing
What to do if my cat has a small hole with pus coming from its ear?
Is your cat's ear hole oozing pus? A Petco vet explains potential infections like abscesses from bites, advising immediate vet care, antibiotics, and recovery.
Updated on September 23, 2025
Petco Pet Education Center
Health & Wellbeing
My male dog's penis is stuck out and his scrotum has a painful rash.
Is your 11-year-old male dog experiencing a painful penis or rash? A certified vet explains potential paraphimosis urgency and why immediate care is vital.
Updated on September 23, 2025
Petco Pet Education Center
Health & Wellbeing
What fish dog food helps dry skin if my dog eats Science Diet?
Is your dog's dry skin a concern? A certified vet offers expert advice on finding the best fish-based dog food alternatives to Science Diet for essential oils.
Updated on February 04, 2026
Petco Pet Education Center
Health & Wellbeing
Dog licking paws, fur loss, and jaw wound: Could it be allergies?
Is your Shih-Tzu crossbreed biting his feet, losing fur, or has a wound? A trusted vet explains these classic allergy signs and advises on treatments like Apoquel or medicated shampoo.
Updated on September 23, 2025
Petco Pet Education Center
Food & Nutrition
Is there a chart for dog food protein biological value and digestibility?
Dog food protein's biological value & digestibility: A vet explains why data is complex. Learn to judge quality by company reputation and your dog's health.
Updated on September 23, 2025
Petco Pet Education Center
Health & Wellbeing
Why does my puppy have bloody, mucousy stools despite diet changes?
Your 4-month-old puppy suffers from ongoing loose stools with mucus and blood. An expert veterinarian explores potential intestinal parasites and advises further fecal testing. Find answers here!
Updated on September 23, 2025
image
Have A Vet Question?

Book an appointment with the pros – our expert vets are here to help.

Book Now