Benadryl (diphenhydramine without any other additives like decongestants) can be given at 1 mg per pound of body weight. (Dr. Cheng on the Ask a Vet board recommends that dosage.) If your dog is wheezing, you may need to get a prescription medication from your vet.
Benadryl, Sominex, and Waldryl are best. Used for relief of allergies, insect bites, and itching. DO NOT USE FOR SNAKE BITES. 2mg per pound of dog every 8-12 hours.