Benadryl (diphenhydramine) is a safe over-the-counter medication you can give your dog. Learn the appropriate dosage, uses, and side effects.

By Kate Eldredge Basedow, LVT
August 24, 2020
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More Than Words Photography by Alisa Brouwer / Getty

Diphenhydramine, widely known by the brand name Benadryl, is an antihistamine that blocks H1 receptors. While drug chemistry may not be your favorite hobby, good old Benadryl may come to the rescue for your dog in a couple of situations. Can you give a dog Benadryl? Yes, it is perfectly safe when given at the correct dosage.

Can You Give a Dog Benadryl for Itching?

Yes, you can! Bug bite reactions and environmental allergies are two of the most common reasons to give Benadryl to dogs. Benadryl can also be helpful for anxiety about events such as thunderstorms or fireworks, for motion sickness, or as part of treatment for mast cell tumors. If your dog contracts heartworms (don’t forget that monthly preventive!), Benadryl may be given to help lessen the risk of reactions to treatment.

For allergies, Benadryl is most effective if given before your dog is exposed to the thing that sparks his allergy. It’s obviously difficult to plan for nasty bug bites, but if your dog has seasonal allergies, you can start giving Benadryl at the time of year when she usually has issues. If it’s being used for thunderstorm anxiety or motion sickness, give the Benadryl about a half hour to an hour ahead of the event so it can start working before your dog becomes stressed or nauseous.

How Much Benadryl Can You Give a Dog?

Thankfully, diphenhydramine has a fairly wide safety margin in dogs. A safe and easy dosage to remember is 1 milligram per pound of body weight. For example, a 50-pound dog would get 50 milligrams of Benadryl. Brand name and generic forms generally come in 25-milligram tablets, so that 50-pound dog would get two tablets at a time. If your dog’s weight is in between a multiple of 25, you can break tablets in half to get the right dosage.

Children’s liquid Benadryl can be used for small dogs using the same dosage. Avoid the adult formula of liquid Benadryl, as this often contains alcohol, which is toxic to dogs. As with any flavored human medication, always check the inactive ingredients for xylitol, a sweetener that is extremely toxic to dogs. Do not give your pet any medication (or any other substance) containing xylitol.

How Often Can You Give a Dog Benadryl?

Benadryl is most commonly given two or three times a day.

Can You Give Benadryl Without Talking To Your Vet First?

It’s always best to consult with your veterinarian before giving your pet any medication or supplement. You want to make sure it’s appropriate for your dog’s symptoms, that you are giving the right dose, and that it will not interfere with other medications your dog is taking. But as long as you know your dog’s current weight and double-check your math on that 1-milligram-per-pound dosage, when you see your dog’s face swelling up from a bug bite you can go ahead and give the Benadryl.

Do not give Benadryl if your dog has a heart condition (particularly high blood pressure), glaucoma, seizures, bladder issues (particularly an obstruction), or lung disease.

If symptoms persist even after giving Benadryl, contact your vet. Swelling of the throat is an emergency and requires immediate intervention with injectable medications that will work faster than a pill given by mouth.

If you find yourself giving Benadryl to your dog on a regular basis, discuss the symptoms you see and the frequency with your veterinarian. While Benadryl may be working well to address the symptoms, there may be other medications that can more effectively address the underlying cause of those symptoms.

What Is the Best Way to Give Benadryl?

Benadryl can be given with or without food. If your dog drools excessively or vomits after being given Benadryl without food, try giving it with some food the next time.

What Are the Side Effects of Benadryl in Dogs?

Potential side effects include:

  • Drowsiness (this may lessen over time if giving Benadryl frequently)
  • Drooling
  • Dry mouth
  • Increased heart rate
  • Urinary retention
  • Less commonly diarrhea, vomiting, or lack of appetite

It’s possible to overdose a dog with Benadryl. If you catch your dog with an open bottle of Benadryl, contact your vet or a pet poison control center to determine the best course of action. Inducing vomiting is often effective if the pills have been consumed recently. 

Signs of a Benadryl overdose include:

  • Agitation
  • Constipation
  • Dilated pupils
  • Rapid heart rate
  • Seizures
  • Slow, shallow, or stopped breathing

What Are Some Alternatives To Benadryl?

Other over-the-counter medications that can be given to dogs for allergies include cetirizine (Zyrtec) and loratadine (Claritin). The supplement quercetin can also be effective, although there has been little research to determine optimal dosing.

Benadryl is a very safe option for sudden-onset and intermittent allergies, and it may be useful for other conditions as well. However, if your dog has chronic issues that require Benadryl, it’s worth having a conversation with your vet to find out if there is a different medication or treatment that can address the cause of your dog’s symptoms.