Food allergy is a common concern for pet owners, however, true food allergy is not very common in dogs and cats and is frequently over-diagnosed. True food allergies account for only 10% of all pet allergies and affected pets shows characteristic signs such as severe itching, hives, skin breakouts, etc. The most common adverse food reaction is called food intolerance and usually causes gastrointestinal upset. It is not an allergic reaction to the ingredients in a food and as such there are no characteristic "allergy" signs. Instead, it is an intolerance to a component of the food—such as the quality of the ingredients, artificial dyes, preservatives or other additives—and causes vomiting and diarrhea.
However, whenever a pet experiences vomiting or diarrhea from a food, owners and veterinarians alike are quick to blame "food allergy". This diagnosis is often inaccurate and many pets are placed unnecessarily on hypoallergenic diets, when potentially all they needed was a high-quality pet food without artificial ingredients. If your pet experiences vomiting or diarrhea when starting a new food, first make sure you are transitioning properly and then consider that your pet may be experiencing an intolerance to inferior ingredients. At that point, you can select a high quality diet without artificial ingredients or consider home-cooking for a short time. Home-cooked diets provide the advantage of being preservative- and other additives-free. If your pet does well on a home-cooked food you can confirm it is not an allergy to the proteins contained in the food—that instead, it is an intolerance to some component of the commercial pet food.
True food allergies do occur and if you still suspect your pet may be affected, contact your veterinarian to discuss a proper food trial. The gold standard of diagnosis is an 8 to 12 week home-cooked unique (novel) protein and carbohydrate food trial in order to avoid possible allergens to which your pet has previously been exposed. Other forms of allergy testing, such as blood and skin tests, are not reliable for diagnosing food allergy. Although you will get results from these tests, they don't accurately correlate with food allergies present in either the dog nor cat and are not recommended by board-certified dermatologists at this time. Contact your veterinarian for more information.