The main sign of allergies in a dog is itching and skin problems. Dogs can become allergic to things in the environment like pollen or dust mites or to foods. These are called allergens.
When the body is exposed to allergens, it releases histamines. These attach to nasal or skin cells and cause them to swell and produce discharge. Antihistamines work by preventing production of histamine.
Side effects of antihistamines include drowsiness, vomiting, diarrhoea, blurred vision and dry mouth. These effects depend upon the dosage and potency of the different types of antihistamines.
A common antihistamine used for dogs is diphenhydramine, the same drug as used in humans. The usual dose is 1 mg for every lb. of body weight twice daily.
Combining with Steroids
Occasionally antihistamines need to be combined with steroids to help control the allergic reaction. Steroids can be hard on the body so they are normally used only for short periods of time.
Antihistamines should not be given to dogs with glaucoma, thyroid disease or heart problems unless under the supervision of a veterinarian.