Diphenhydramine (Benadryl) for Dogs

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 Used for in Dogs?

Diphenhydramine, an antihistamine found in products like Benadryl and Vetadryl (formulated for pets), is commonly used to treat allergic conditions in dogs, including hives, insect bite reactions, and allergic skin disease. It’s available in various forms, from oral to topical. 

While some pet parents use Benadryl to calm dogs during thunderstorms or fireworks, it’s not typically effective for anxiety. If your dog is experiencing stress or noise phobias, consult your veterinarian about more appropriate and effective treatment options. 

Is Diphenhydramine FDA-Approved? 

Benadryl 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.  

Speak with your veterinarian about whether diphenhydramine is right for your dog. 

Compounded Diphenhydramine 

In certain circumstances, your vet may recommend a compounded formulation of diphenhydramine. 

Compounded medications are prescribed, for example, 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 

• The pet is allergic to an ingredient in the FDA-approved medication  

Compounded medications are not FDA-approved. They are formulated by your veterinarian and dispensed either by a licensed pharmacist or veterinarian on an individual basis to best suit a patient’s particular needs.  

Diphenhydramine Considerations for Dogs

Diphenhydramine should not be used in dogs with certain health conditions or those taking specific medications. It's important to talk to your veterinarian before giving your dog this medication to ensure it’s safe based on their health history. 

Avoid giving diphenhydramine to dogs who: 

• Are allergic to it or other antihistamines 

• Have glaucoma, prostate disease, urinary obstruction, or high blood pressure 

• Are pregnant or nursing 

Giving diphenhydramine with certain medications can result in health risks to your dog, so it’s important to discuss your dog’s medications, including vitamins and supplements, and medical conditions with your veterinarian. 

How Diphenhydramine Works in Dogs

Diphenhydramine is an antihistamine that works by blocking histamine receptors in the body. Histamine is a chemical released during allergic reactions and is responsible for symptoms like itching, sneezing, swelling, and hives.  

Although it doesn’t stop histamine from being released, diphenhydramine prevents the body from reacting to it.  

How To Give Diphenhydramine to Dogs

Follow the directions on the drug label or as provided by your veterinarian. Your veterinarian will prescribe a dosage based on your dog’s weight.  

Missed a Dose? 

Speak with your veterinarian about what to do if you forget to give a dose of diphenhydramine. 

Generally, your vet may instruct you to give it when you remember, or if it is almost time for your dog’s next dose, to skip the missed dose and resume your normal dosing schedule. 

Do not give extra or double doses. 

Possible Side Effects of Diphenhydramine in Dogs

According to the Vetadryl label, possible side effects include: 

• Sleepiness (lethargy) 

• Dry mouth  

• Trouble peeing (straining to urinate) 

• Diarrhea  

• Vomiting  

• Decreased appetite 

Human Side Effects 

Due to possible side effects, humans should never use medicine dispensed for their pets and pets should not be given any medicine dispensed for a human’s use. Diphenhydramine frequently has dosages different from those prescribed for your pet by a veterinarian. 

If you accidentally ingest a pet medication, seek medical attention in person or call Poison Control at 800-222-1222. 

Monitoring Your Pet 

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

When To Call Your Vet 

Call your veterinarian if you see any of the following, or if you have additional questions or concerns about the use of diphenhydramine: 

• Severe side effects (above) 

• Your pet’s condition worsens or does not improve with treatment 

• You see or suspect an overdose 

Diphenhydramine Overdose Concerns in Dogs

An 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 

• Death 

If you suspect an overdose, immediately seek emergency veterinary care or contact an animal poison control center. Consultation fees often apply. 

• Pet Poison Helpline 855-764-7661  

• ASPCA Animal Poison Control 888-426-4435  

How To Store Diphenhydramine for Dogs

Keep these storage tips in mind for diphenhydramine: 

• Most products are stored at controlled room temperature. Vetadryl is recommended to be stored between 59–86 F (15–30 C). Storage requirements vary between formulations, so check the product packaging for storage information. 

• Compounded medications should be stored according to the compounding pharmacy’s label. 

• Keep the container tightly closed to protect from moisture and light. 

• Keep out of reach of children and pets.