Antihistamines are a class of drug that decrease inflammation by inhibiting histamine release. There are many drugs in this class including diphenhydramine (Benadryl), hydroxyzine (Atarax), chlorpheniramine (Chlortrimeton) and cetirizine (Zyrtec). The most common side effect is sedation, dogs sleep more than normal. This effect usually improves after a few days of therapy. In rare individuals, the opposite reaction occurs with dogs trembling, salivating, panting and showing other signs of excitation, although this is far more common in cats.
The problem with antihistamines is that not all dogs respond to them. Most will improve somewhat, but not completely. A few show no response at all. I used diphenhydramine on my own dog Susie. It worked well for two years and then she stopped responding. I switched to chlorpheniramine until it stopped working, then switched back to diphenhydramine. I recommend trying a specific antihistamine for 7 days before switching to another. I also like to combine antihistamines with other treatments to give the dog more relief such as omega 3 fatty acids, shampoo and creme rinse. For emergency situations, when your dog is keeping you up all night scratching and licking their abdomen, I have owners spray Dermacool on the area. This product contains lidocaine which is a topical anesthetic. After a few minutes, the spray numbs the area allowing the dog and their owner to go back to sleep.
I find that antihistamines are more effective when given early, before the allergic reaction gets too strong. I teach owners how to spot allergies in the early stages. In many dogs, the ears are one of the easiest places to spot the onset of allergies. Faint red streaks occur on the inside of the ear flap. It often takes only a few days of therapy to control a dog whose condition is caught at this stage. Early detection also prevents the inflammation from transforming into a full blown ear infection.
Regardless of whether your dog suffers from allergies or not, I recommend that all pet owners keep an antihistamine on hand just in case your pet is stung by a bee or another insect. This can be a life-threatening condition. Before a crisis unfolds, call your veterinarian to get the proper type and dose of antihistamine to use in an emergency situation. Early treatment is the key to successful treatment of allergies.