Diphenhydramine (Benadryl®, Vetadryl®)

PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet.

What Is Diphenhydramine (Benadryl®)?

Benadryl^® is an antihistamine used to manage allergies in people. The active ingredient, diphenhydramine, is also commonly used to treat allergies in dogs, cats, horses, and other animals.

It is available in a wide variety of products, ranging from oral to topical medications. Although Benadryl® is the branded version made for humans, Bendryl® is currently not FDA approved as a veterinary medication. However, the active ingredient, diphenhydramine is readily utilized in the veterinary field, and veterinarians can legally prescribe certain human drugs in animals in certain circumstances. This is called extra-label, or sometimes off-label use, because this use isn’t described on the drug label. There are also diphenhydramine products made specifically for animals, such as Vetadryl®.

Diphenhydramine has been used for years in animals to manage allergic conditions such as hives, allergic reactions to insect bites and stings, and allergic skin disease. The affordable cost and relative safety of diphenhydramine allow it to be a commonly prescribed medication in animals with allergies, especially in combination with other treatments. 

Some pet parents give Benadryl^® to calm anxious dogs during thunderstorms or fireworks. However, Benadryl^® is not typically an effective treatment for anxiety. Talk to your veterinarian about your dog’s behavior. More effective medications are available to treat anxiety and noise phobias in dogs.  

In certain circumstances, your vet may recommend a compounded formulation of diphenhydramine. Compounded medications are prescribed if there’s a specific reason your pet’s health can’t be managed by an FDA-approved drug, such as if your pet has trouble taking pills in capsule form, the  dosage strength  is not commercially available, or the pet is allergic to an ingredient in the FDA-approved medication. Compounded medications are not FDA-approved. They are created by either a veterinarian or a licensed pharmacist on an individual basis to best suit a patient’s particular needs. You can learn more about compounded medications here.

How Diphenhydramine (Benadryl®) Works

Diphenhydramine (Benadryl^®) is an antihistamine that works by blocking histamine receptors. Histamine is a natural chemical that is released in a pet’s body during an allergic reaction. The classic signs of allergies, such as itching, sneezing, swelling, and hives, are due in part to the action of histamine. Diphenhydramine does not block the release of histamines; instead, it blocks some of the body’s ability to respond to it. 

Histamine can also trigger vomiting. By blocking histamine receptors, diphenhydramine may also help reduce vomiting triggered by motion sickness in dogs. 

Diphenhydramine (Benadryl®) Directions

Follow the directions provided by your veterinarian. Your veterinarian will prescribe a dosage  based on your pet’s weight. This dosing for animals will likely differ from human dosing recommendations found on the label of over-the-counter diphenhydramine.  

Missed a Dose? 

If you forget a dose, give it when you remember and resume your dosing schedule. (For example, if you are giving diphenhydramine every 12 hours, give the missed dose then wait 12 hours before giving the next dose.) If it is close to the time for the next treatment, you can skip the missed dose to resume your normal schedule. Do not give two doses at once or extra doses unless directed by your veterinarian. 

Diphenhydramine (Benadryl®) Possible Side Effects

According to the drug label Vetadryl^®, side effects can include:

• Sedation 

• Decreased energy level (lethargy) 

• Constipation 

• Dry mouth 

• Urinary retention 

• Diarrhea 

• Vomiting 

• Decreased appetite 

• Excitement, especially in cats

Human Side Effects

While this medication can also be used in humans, dosing for pets is often different and it may have different side effects. If you accidentally ingest your pets medication, call your physician or local poison control center.

Monitoring

No specific monitoring is required for this medication, but your veterinarian may recommend routine testing depending on your pets' individual needs, other medications they may be on and/or the issue that initially caused your pet to be placed on this medication.

Call Your Vet If: 

• Severe side effects are seen (see above) 

• You see or suspect an overdose 

• You have additional questions or concerns about the use of diphenhydramine 

Call your vet if side effects are seen (see above) or if you see or suspect an overdose. Call your vet or pharmacist if you have you have additional questions or concerns about the use of diphenhydramine. 

Diphenhydramine (Benadryl®) Overdose Information

Overdose of diphenhydramine can either excite or depress the nervous system. Signs include hyperactivity, agitation, muscle tremors, fever, incoordination, increased heart rate, decreased energy (lethargy), slowed or difficult breathing, seizures, coma, and death. Emergency treatment is often necessary after an overdose. Always store the medication in a secure location out of reach of all pets and children. 

If you suspect an overdose, immediately contact your veterinarian or an animal poison control center. Consultation fees often apply. 

Pet Poison Helpline (855) 764-7661 

ASPCA Animal Poison Control (888) 426-4435 

Diphenhydramine (Benadryl®) Storage

Storage requirements vary between formulations. Check the product packaging for storage information. Most products are stored at controlled room temperature. Vetadryl^® is recommended to be stored between 59-86°F. Keep the container tightly closed in order to protect from moisture and light.

Keep out of reach of children and pets.

Diphenhydramine (Benadryl®) FAQs

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