Diphenhydramine for Dogs and Cats

Dosage Forms Available:

  • Capsule
  • Transdermal Gel
  • Injection
  • Oral Liquid


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Diphenhydramine is an antihistamine for dogs, cats, and other animals generally used to treat allergy symptoms but may be used for other medicinal purposes.

What is Diphenhydramine (Benadryl)?

Diphenhydramine for dogs and cats is an antihistamine treatment. Antihistamine medications work to block H-1 receptors which trigger allergy symptoms to small blood vessels and muscles. Inflammation occurs when histamine attaches to these receptors. 

Humans use Benadryl for allergy symptoms, and under the direction of a veterinarian, so can pets. Animals having the same allergy symptoms as humans doesn’t necessarily mean that the pet has allergies, so be sure to consult with the veterinarian first before giving cats and dogs Benadryl.

Benadryl for dogs and cats may also be used for insect stings, travel sickness, and calming nerves. Diphenhydramine for dogs and cats is also useful prior to a vaccine injection if a temporary adverse reaction is expected.

Diphenhydramine Side Effects

Diphenhydramine for dogs and cats is generally safe, some side effects could emerge. Use this list to be aware of the possible side effects associated with diphenhydramine.

Common Side Effects

  • Excitability (especially in cats)
  • Odd behavior
  • Lethargy, sedation, or depression (tension)
  • Dry mouth or drooling.
  • Increased breathing rate
  • Decrease in urination

Less Common Side Effects

  • Diarrhea
  • Vomiting
  • Decreased appetite 
  • Rapid heartbeat
  • Labored breathing

Diphenhydramine Precautions

As with any medication, there are precautions to heed when administering diphenhydramine for dogs and cats.

Preexisting conditions should be assessed before diphenhydramine is given to an animal. A complete medical history should be given to the veterinarian to determine if this medication is appropriate. Not all will respond to this drug the same way, so the dose may need to be adjusted for some animals.

Laboratory testing determined that Benadryl for animals can cause birth defects. If giving high doses of diphenhydramine to a nursing mother, the drug is passed into the milk. For this reason, it is recommended that diphenhydramine only be given when there isn’t an alternative.

This drug should not be taken while an animal undergoes skin allergy testing as this would skew the results. Cease medication two weeks prior to testing.

Animals Who Should Not Use Diphenhydramine 

  • Puppies or kittens.
  • Dogs and cats allergic to similar antihistamines.

Situations That Require Caution When Giving Diphenhydramine 

  • Pregnant or lactating animals
  • COPD
  • Heart disease
  • Glaucoma
  • Prostate enlargement
  • Intestinal, bladder neck obstruction, or pyeloduodenal obstruction
  • Bladder disorders
  • Prostatic hypertrophy
  • Hyperthyroidism
  • Seizure disorders
  • High blood pressure
  • Hypertension
  • Heart failure
  • Geriatric animals
  • Working dogs

Possible Interactions

Problems may occur when diphenhydramine is taken at the same time as these medications:

  • Amitraz
  • Anticholinergic drugs
  • CNS depressant agents (sedatives, anesthetics, or pain medications)
  • Epinephrine
  • Furazolidone
  • Heparin sodium or calcium
  • Selegiline
  • Tricyclic antidepressants
  • Warfarin sodium


Benadryl should be stored at room temperature. Do not freeze.


The dosage amount of diphenhydramine varies depending on the way it will be administered. For animals who have difficulty swallowing pills, pharmacy compounding or flavoring may help. 

Standard Dosage by Method

Capsule or tablet: 12.5 mg, 25 mg (most common), or 50 mg.

Liquid form: 12.5 mg per 5 ml, or 25 mg per 5 ml.

Injection: 50 mg per ml.

Diphenhydramine for Dogs Dosage

These are general guidelines with diphenhydramine dosage for dogs. Always follow your veterinarian’s explicit directions.

For allergies: 2-4 mL per kg of body weight to be given orally every 8-hours

For allergic reactions: 0.5-2 mL per kg of body weight to be given by injection.

For nausea or motion-sickness: 2-4 mL per kg orally or intramuscularly every 8 hours

Diphenhydramine for Cats Dosage

These are general guidelines for administering diphenhydramine for cats. Always follow your veterinarian’s explicit directions.

For allergies: 2-4 mL per kg of body weight by mouth every 8-12 hours. 

For allergic reactions: 0.5 — 2 mL per kg of body weight by injection.

Missed Dose

In the case of a missed dose, stay on the regular schedule as best you can. Skip the missed dose if it’s almost time for the next dose, and continue with the regular schedule. Avoid giving your pet more than one dose at a time or too close together.

Signs of Diphenhydramine Overdose

As with any medication, there are risks if an overdose occurs. Be aware of the signs of an overdose so you can take immediate action. If you notice any signs of an adverse reaction or overdose, call the veterinarian or emergency contact immediately.

Overdoses can be prevented by communicating with other caretakers to make sure that a dose hasn’t already been given. Also, keep medications away from your pets. An animal may help themselves to it, especially if it has been flavored with compounding.

Signs of an overdose are:

  • Agitation
  • Seizures
  • Shaking or shivering
  • Rapid heartbeat
  • Dilated pupils
  • Fainting or extreme drowsiness
  • Constipation or vomiting


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