If your dog has recently started eating a new food, one or more of the ingredients in it could cause an allergic reaction. A food allergy can be triggered by any protein-containing ingredient found in a new diet, including a meat, vegetable or carbohydrate. But ingredients in the same old diet he's eaten for years can trigger allergies all of a sudden, too.
In order to experience a true food allergy, your dog must have been fed his new diet or the offending ingredients in it more than one time, usually over a period of a few weeks or months. A new ingredient in a recent dietary change might not be responsible for your pup's allergies. The cause could be an ingredient in his old diet that's also contained in his new diet.
Diagnosis and Treatment
To diagnose food allergy, your vet will put Fido on a hypoallergenic diet, containing limited ingredients he's never eaten. If he responds favorably within 12 weeks, it's likely he was allergic to what you were previously feeding him.
Don't confuse a food allergy with food intolerance, which results in stomach upset when you switch foods abruptly. Food intolerance, a digestive issue, will clear up within days.