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Reactions to Antihistamines in Dogs

| Updated September 26, 2017

Allergies are among the most common medical complaints in dogs. While most humans react to allergens with sneezing and coughing, dogs are more likely to develop skin problems. One common treatment for allergens in both species is antihistamines. It's important to know which ones to use and what the possible side effects can be.


Allergens cause problems because the immune system recognizes them as a threat to the body. This usually happens because of oversensitization, whether to an airborne allergen or to a type of food. Once the allergen enters your dog's body, the immune system attacks mast cells in the skin, causing a release of histamines and the accompanying symptoms. Antihistamines can prevent this release, providing some relief.


The most common antihistamine given to dogs is Benadryl, also called diphendryamine. Many vets use a dose of 1 mg per pound of the dog's weight. Other options include clemastine, chlorpheniramine and hydroxyzine. Hydroxyzine is a very strong antihistamine and may cause more serious side effects than weaker types.


According to Vet Info, about 15 to 20 percent of dogs benefit from the use of antihistamines alone. This number increases to 25 to 40 percent when antihistamines are used in conjunction with fatty acid inhibitors. Vets recommend that you try several antihistamines, since each pet's reaction will be a little different.


Not all antihistamines are right for your dog. Side effects of antihistamines in dogs include drowsiness and lethargy. Some dogs also get diarrhea or nausea. A few will become excitable and nervous when given certain antihistamine medications. You should avoid these. If your dog does become drowsy or tired when you give it antihistamines consider administering them at night, when your pet would normally be sleeping.


One common problem for dog owners who treat their pets with antihistamines is determining the right dose. Always check with your vet before dosing your dog, even if you have the medication in your home. One of the most common difficulties for dogs treated with antihistamines is that the initial dose is too low. Each drug has its own dosing requirements. Follow instructions carefully when administering medication to your pet.