Benadryl is the brand name for the active ingredient diphenhydramine HCL. Diphenhydramine is a first-generation ethanolamine-derivative antihistamine, which is the scientific way of classifying antihistamines that can cross the blood-brain barrier from those that cannot. The ability to cross the blood-brain barrier makes them very effective, but also increases the risk of adverse effects when compared to less effective second-generation antihistamines. While Benadryl is not yet FDA-approved for veterinary use, it is considered safe for use in dogs and cats and is commonly used in veterinary practices across the U.S.

Diphenhydramine is a receptor antagonist, which means that the drug works by blocking the receptors that receive histamines in the body. This relieves many of the symptoms associated with allergies, like itching, sneezing, and hives. The body still produces histamines, but the receptor antagonist blocks the receptors from registering the histamines. It is a bit like the mail-person trying to deliver mail to an already full mailbox. The letter arrives, but there is no room for it.Benadryl is a great medication for use in dogs with mild-to-moderate allergies. Seasonal allergies, food allergies, environmental allergies, and allergic reactions to snake and insect bites all respond to Benadryl in most cases. Benadryl is commonly used to treat itchiness in dogs caused by skin allergies, and it also reduces many of the other symptoms of allergies, including:

Itching
Hives
Swelling and inflammation
Redness
Runny nose and eyes
Coughing
Sneezing
Anaphylactic reaction One of the side effects of Benadryl is drowsiness, which helps to calm anxious dogs. The Merck Veterinary Manual states that diphenhydramine may relieve symptoms of mild-to-moderate anxiety in pets associated with travel. It also may help relieve motion sickness during car rides and plane rides.

Veterinarians prescribe Benadryl for dogs with mast cell tumors to help mitigate the effects of the massive histamine release caused by mast cell degranulation. Benadryl is also used as adjunct therapy for other conditions. Veterinarians sometimes prescribe diphenhydramine during heartworm treatment, as it helps prevent allergic reactions associated with heartworm treatment therapy.

Benadryl makes an excellent addition to your pet emergency kit. If you don’t already have a pet emergency kit or pet travel kit, consider putting one together today.

When to Ask Your Vet About Benadryl for Dogs
Before you reach for the Benadryl, consult your veterinarian about your dog’s symptoms. Allergy symptoms like itching and red eyes are also signs of more serious conditions. In some cases, like glaucoma, giving your dog Benadryl can actually worsen your dog’s condition. Red, goopy eyes could be a symptom of allergies, or it could also be a sign of an eye disease like glaucoma or dry eye, which Benadryl will not help treat. Similarly, itching is frequently associated with both allergies and other skin conditions. As Benadryl is ineffective for treating certain skin diseases, it is always a good idea to consult with your veterinarian to make sure you are doing the best thing for your dog’s health.

Your vet may recommend you bring your dog in for a checkup. If you choose not to bring your dog in against your veterinarian’s advice, or if you administer Benadryl without first consulting your veterinarian, be sure to keep a close eye on your dog and call your vet if your pet’s condition worsens.There are side effects associated with using Benadryl for dogs that all dog owners should be aware of. Just like people check with their doctors before taking a new medication, you should always check with your veterinarian before introducing Benadryl to see if it has any potential drug interactions with your dog’s other medications, or if it could worsen a preexisting condition.

If your dog has any of the following conditions, only use Benadryl after consulting your veterinarian:

Angle closure glaucoma
Severe heart failure
Prostatic hypertrophy
Bladder neck obstruction
Seizure disorders
Hypertension
Allergic lung disease
Pregnancy
Common side effects associated with using Benadryl for dogs include:

Sedation
Dry mouth
Urinary retention
Hypersalivation
Increased heart rate
Rapid breathingThe best way to determine the correct Benadryl dosage for dogs is to consult your veterinarian. The Merck Veterinary Manual recommends administering 2-4mg of Benadryl per kilogram of body weight, two to three times a day. However, this dosage can vary depending on your dog’s existing medical conditions.

Never use time-release capsules for dogs, as capsules are absorbed differently in dogs than in humans and may affect your dog’s dosage. They may also break open when chewed and deliver too much medication at one time, putting your dog at risk of an overdose. If you choose to use a liquid Benadryl, it is safer to use a children’s liquid formula, as most do not contain alcohol (although they do contain sodium). Children’s Benadryl pills or tablets can also be used to dose very small dogs. Dosage for liquid Benadryl is different than the dosage for Benadryl pills. Consult your veterinarian for the appropriate dosage, and use a syringe to increase measurement accuracy and ease of administration.

Benadryl typically takes 30 minutes to start working, so plan accordingly if you plan on using it to treat anxiety or mild motion sickness. For dogs with chronic allergies or conditions that require daily doses, consult your veterinarian about the appropriate dosage, as it may change over time.

Always consult your veterinarian before giving Benadryl to pregnant or nursing dogs, since the drug is not recommended for use in these animals.