Well, this is primarily a forum for human health questions, but since I'm a dog lover, I'll answer your canine conundrum.
Are you sure the problem is allergies? The symptoms of allergies (such as nasal allergies or hay fever) are very different in dogs than in humans. If you have any questions at all about whether or not your dog is truly suffering from allergies, you need to consult a veterinarian.
My understanding (but don't take that to the bank) is that Zyrtec (generic name: cetirizine) is safe for dogs. I don't know the dosage, however. You can call a 24-hour animal hospital, and they will probably be happy to tell you.
Diphenhydramine (brand name: Benadryl) is safe for use in dogs. It may be a better choice than cetirizine, in fact. But you should consult a veterinarian for specific dosing instructions for your dog.
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