Antihistamines help to control itching in approximately 1/4 to 1/3 of dogs with skin allergies, especially if started when skin itch is still mild and if used with Omega-3 Fatty Acids (see below). They are inexpensive, readily available as over-the-counter medications...
Why Choosing the Right Antihistamine for Dogs Is Not StraightforwardThe 7 Most Commonly Used Antihistamines for DogsQuick Reference: Antihistamine Dosage Chart for Dogs
I would be hesitant to use antihistamines in dogs that have respiratory conditions such as asthma or bronchitis, liver disease, and seizure disorders such as epilepsy. Keeping in mind that antihistamines can be sedating, avoid them during planned strenuous activities, such as swimming.

Antihistamines for dogs: Antihistamines are commonly used to relieve itching and other allergic symptoms in dogs.Some common antihistamines for dogs include: Zyrtec (Cetirizine): A popular & safe antihistamine that can be effective in reducing itching and inflammation in dogs.
Causes of Dog Rash. While searching online, its easy to find many pictures of dogs with skin rashes or lesions.Skin supplements formulated for dogs. Antihistamines such as Benadryl. Check with your vet to see if this is right for your dog and what would be a safe dose.

Clemastine for dog dog allergies. Like hydroxyzine, clemastine is a prescription-strength antihistamine for dogs. Its more effective than Benadryl in many cases of canine allergies, but not as effective as more targeted medications.