Allergies aren't fun for anyone, but especially not for your dog who can't tell you what's making him so sick. If your dog vomits frequently, has diarrhea, irritated skin, a poor coat condition or hair loss, then he may have a food allergy.
It may take months or years before your dog develops an allergic response to a particular food. However, once he's allergic, he will almost always have a negative reaction to that food.
Allergic reactions are most commonly associated with protein sources - usually the meat in your dog's food. If your dog has an allergic reaction to a certain meat, you may want to try a food with a new protein source - new to your dog, that is - such as egg, duck, salmon, lamb or whitefish.
If none of this helps, your dog may be allergic to all of these proteins and will need a food with specially broken-down proteins. Visiting your veterinarian will help make sure you and your dog are on the right track.
For an accurate diagnosis and treatment options, always consult your veterinarian.