The most common allergies in dogs are flea allergies (scratching at hindquarters) then environmental allergies (atopic dermatitis), and then food allergies. You need to rule out flea allergies first! And this means a good flea control like Capstar orally every day. Give this 8 weeks and see if the dog gets better. You also need to get rid of fleas still in the environment. Advantage, or Frontline are not good enough flea preventatives for a flea allergic dog. You don’t even need to see the fleas on the dog, just one bite can cause a reaction and extreme itching for up to 2 weeks after.
Then you can try a food trial. You should go with something like Royal Canin Rabbit and Potato or Iam’s kangaroo and oat. The most common food allergies in dogs are beef, chicken, wheat and dairy. Don’t feed over the counter brands like Blue Buffalo, Taste of the Wild or Dick Van Patton’s even if they say Rabbit on the label, their plants process other meats, like chicken and beef on the same machines. Royal Canin limited ingredient diets do not. Diets like venison cross react with beef, and duck cross reacts with chicken, so don’t use those diets. When doing a food trial you need to do it for 8 weeks. You need to not give any other treats, no medications that are flavored (like heartguard, rimadyl, comfortis etc…), no flavored toys, no rawhides, no people food. Even minced rabbit may have been processed on the same machines as beef or chicken, so don’t give any people food. You need to do this for 8 weeks and see if there is an improvement. Unfortunately, there is no cure, but all itchy dogs can be managed with great success if you stick to it!
So you have been strict on flea control, and strict on the food diet, and your dog is still itching? Then most likely it is an environmental allergen (atopic dermatitis) that can be caused by pollen, mold spores, dust mites etc…5 ways of treating this 1) allergy testing and immunotherapy via injections 2) Atopica 3) Steroids 4) over the counter antihistamines 5) fatty acids…