For chronic chewing, your dog needs to see an allergist, pronto. My little chihauhau was chewing the fur from his little feet, and had yeast infections, plus bacterial.
We tried steroidal meds he put on 5 pounds, yes doubled his weight and it is bad for them. It didnt work, next step was to put him on perscription dog food, and see an allergist. The allergist did the testing like on a human. Found he was allergic to many many things. Cats being one, I thought that was cute. I now give him a weekly injection, it was expensive but when you take on a companion you contract to care for them.
Benadryl is safe for dogs, make sure its not the cold kind, just the allergy kind. Also when a dog chews his feet, and has hair loss on the underside it is a sign of inhaled allergies. Like pine trees, pollen, dust mites etc... almost like hay fever in humans.