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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, it is often used in animals. 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.  

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

  • Sedation 

  • Decreased energy level (lethargy) 

  • Constipation 

  • Dry mouth 

  • Urinary retention 

  • Diarrhea 

  • Vomiting 

  • Decreased appetite 

  • Excitement, especially in cats 

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. 

Benadryl® tablets: Store between 68-77°F (20-25°C). Protect from light and moisture.  

Vetadryl®: Store between 59-86°F (15-30°C).  

Diphenhydramine (Benadryl®) FAQs

Can dogs have Benadryl® for bee stings? 

The antihistamine effects of Benadryl® can help treat allergic reactions to bee stings. However, if your dog is having a serious allergic reaction, emergency veterinary care and treatment with more powerful medications are often necessary. Signs of a serious allergic reaction include trouble breathing, vomiting, diarrhea, swollen face, hives (multiple raised lumps that suddenly appear along the skin), weakness, collapse, or seizures. Seek emergency treatment immediately if these or any other signs are present.  

Can dogs take Benadryl® liquid?

Some liquid formulations of Benadryl® contain alcohol or xylitol (a sugar alternative that is toxic to pets) and should not be used in animals. Always check with your vet and be sure to check the label before use to ensure that the medication does not contain alcohol or xylitol. 

Talk to your veterinarian about the best way to medicate your dog.  

Is human Benadryl® ok for dogs? (children’s, cream, spray, etc.)

Human formulations of Benadryl® are often used in animals. However, dosages for animals usually differs from dosages used by people. Talk to your veterinarian to get appropriate dosing information for your pet. Also, avoid any products (especially liquids) that contain alcohol or xylitol. 

Is it possible for a dog to have a Benadryl® allergy?

Drug hypersensitivity can rarely occur with any type of medication. Talk to your veterinarian if your pet displays any side effects after using Benadryl® or any other medication. 

What does Benadryl® do to cats?

Your veterinarian may recommend Benadryl® for a cat to help manage allergies, allergic reactions, or travel anxiety. Benadryl® is typically not very effective for managing motion sickness in cats, so alternative drugs are typically used. See above for information about the side effects of Benadryl®. In particular, the drug can unexpectedly cause over excitement and hyperactivity in cats.  

Can Benadryl® kill a cat?

Talk to your veterinarian about an appropriate dose of Benadryl® to use for your cat. The doses routinely recommended by veterinarians are considered safe for cats. However, a large overdosage of Benadryl® could potentially kill a cat.  

How much Benadryl® does it take to sedate a cat?

Talk to your veterinarian if your cat needs to be sedated. Do not use over-the-counter Benadryl® for sedation unless it is recommended by your veterinarian. Alternatively, your veterinarian may recommend a different drug that safely provides more reliable sedation.  

How long does Benadryl® take to kick in for cats?

Benadryl® typically starts working 30-60 minutes after oral administration.  


The information in this article is for educational purposes only and is not a substitute for advice, diagnosis, or treatment from a veterinarian. If you believe your animal is experiencing a medical emergency, call your veterinarian office immediately or seek immediate care from your local animal hospital. 


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