Our one of our adopted dogs is very allergic. He gets benadryl 2x daily but we still have to take him to the vet on occasion for a shot. Depending on the time of year the shot can last 1 month up to about 3 months but then he's back for another shot.
Interestingly enough, when Tramp was adopted by our friend she adopted him because he had almost no hair on his body and his skin was very red and irritated. Turned out he was in a kennel with a concrete floor and, yep, Tramp's allergic to concrete. When my friend lost her farm she had to give up 3 of her 5 dogs and we were blessed Tramp. Our schnauzer was on a special allergy food and when Tramp came along we gave him the same food and low and behold ... he now has hair thick enough he has to go to the groomer. He's a heinz-57 variety and it's really funny because the hair on the back half of his body is more wirey than the soft hair on the front half of his body ... a true mutt!