You need to find out if your dog is really allergic and what he is actually allergic to.
There are some foods that are made from duck meat, others from lamb... and limited ingredients. usually vets will try to figure out by elimination what your dog is allergic to.
Most commonly they will put your dog in one of these special diets , with very different ingredients from regular commercial foods for about 3 months, if all allergies cleared up it is confirmed food allergies, then slowly introduce other ingredients one at a time to find out what the dog is not allergic to. It is a slow process.
There are many prescription foods you can get from your vet or order online with your prescription on hands. Science diet, Purina and Royal Canin, any of these brands make prescription diets for alergies.
Do some research , talk to your vet, if you are not satisfied , get another opinion.
Good luck.
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