There are many causes for dog skin allergies. It takes time and patience to find out what allergens are causing the allergy. This can happen overnight too. One of the common causes for dog skin allergies is exposure to some irritant prevalent in household cleaning products, or laundry products. Even certain dog shampoos may contain the allergen not suiting your pet. This is called contact skin dermatitis. Another trigger for dog skin allergies would be bites from fleas, tick and mites. This would cause discomfort, itchy skin, and skin rash. Dog food can also result in dog skin allergies. But it takes time and patience to narrow down the allergen, and eliminate it totally. Once the known allergen in dog food is identified, then it is easy to provide allergy free dog food.
Symptoms in dog skin allergies manifests as itchy skin, scratching, skin rashes, biting one self till the skin gets sore and hair loss too.
First step would be a correct diagnosis of the problem your pet suffers before an allergy medicine is administered to the dog. In some case a skin disorder and an allergy will have the same symptoms. Consultation with the vet will give a clear picture