Does My Pet Have a Food Allergy or Something Else?

You’ve probably seen posts online that say a food allergy might hit your pet, or ads for pet foods that claim not to trigger allergies. In fact, food allergies don’t bother many pets. If your dog or cat has itchy skin or diarrhea, something else is more likely behind it.

What You Might See

You may start to think that your pet has a food allergy if you see that your pet:

  • Itches a lot around the ears, has itchy or runny eyes
  • Gets hives on the skin or has red and inflamed skin
  • Has swelling in the face, ears, lips, eyelids or earflaps
  • Vomits or has diarrhea
  • Sneezes
  • Keeps getting ear infections
  • Licks the paws a lot
  • Has gas

Of course you want to help. But a food allergy affects only 1 in 10 pets whose owners think they may have one.

How Do You Know if It’s a Food Allergy?

A food allergy happens because of a pet’s immune system. That’s what normally fights off germs and other things that could cause disease.

When a dog or cat has a food allergy, their immune system mistakes food for something harmful, then goes on the attack. That causes the itchy skin, vomiting, or other symptoms.

There’s only one way to make sure your pet has a food allergy. It’s called a dietary elimination test or food trial. It takes several steps:

  1. Switch your pet to a diet made of things it has never eaten. You might have to buy food from the vet or make it at home. For the next 10-12 weeks, nothing else that’s flavored can go into your pet’s mouth. Not food, not chew treats, not scraps from your dinner table, and not even medications that are flavored. If your pet’s symptoms clear up, that may mean it has a food allergy. But that isn’t settled yet.
  2. Now go back to your pet’s old diet. If the itchy skin or other symptoms come back, your pet probably does have a food allergy. But you’re still not finished.
  3. Switch back to the special diet from the first step and wait for the allergy symptoms to go away again. Now add one ingredient from the kind of food that your pet always ate, and then wait a while. If the symptoms don’t come back, your pet isn’t allergic to that. So add back another ingredient from the old diet. Keep doing that until you get to one that makes the symptoms return. That’s probably the culprit for the allergy.

It takes weeks or months to do all that. So your vet will probably want to start by checking for other conditions that could trigger the same symptoms.


Other Things That Cause Skin Problems

Itching and skin infections most often come from:

  • Fleas
  • Allergies to fleas
  • Allergies to things in the air or in the pet’s surroundings, like dust mites, pollen, or grasses

When fleas or a flea allergy are behind your pet’s skin condition, that’s usually the easiest thing to figure out. If you see fleas, your vet will fix you up with a product that kills them, and possibly also a medication to stop the allergic reaction to fleas. If the symptoms clear up, that’s it.

It’s harder to nail down if your dog or cat is allergic to things like pollen. Your vet will examine your pet, ask you about its health history, and do skin tests.

Once you know what your pet is allergic to, your vet can work out how to handle it. You may have to:

  • Keep your pet away from whatever it’s allergic to, if that’s possible.
  • Help your pet keep its coat clean.
  • See that your pet gets regular allergy shots or whatever medicine your vet prescribes.

Whatever the solution is, you’ll have to make a long-term commitment to stick with it. Your pet will depend on you.

Other Things That Cause Digestive Troubles

Is there some kind of food that just doesn’t agree with you? The same thing can happen with your dog or cat. Maybe it’s eating something that has too much fat, for instance.

Vets call this food intolerance. The difference between this and a food allergy is that food intolerance doesn’t involve the immune system.

To figure out if this is what’s wrong with your pet, you’ll have to follow the same trial-and-error process as you would for a food allergy, described above.

Diarrhea and vomiting can also come from:

  • A virus
  • An infection
  • A parasite
  • A condition in your pet’s digestive system. One example is pancreatitis, which is inflammation in the pancreas, the organ that makes chemicals that digest food.
  • An obstruction. This is when something is stuck in the stomach or intestines.


To figure out what’s wrong with your dog or cat, your vet may need to:

  • Consider your pet's age, past illnesses, other animals it has been around, and other information about its health
  • Do a physical exam
  • Do lab tests on its blood or stool samples
  • Do X-rays or an ultrasound
  • Do a biopsy or other tests on samples of its fluids or tissues

After your vet figures out what’s wrong, your dog or cat may need:

  • Medicine that kills germs or parasites
  • Drugs to help with diarrhea or vomiting
  • Fluids to replace what it has lost because of diarrhea
  • Surgery
  • Pain medicine


WebMD Veterinary Reference Reviewed by Amy Flowers, DVM on August 22, 2020



Cummings Veterinary Medical Center.

American Kennel Club.

Your Dog (Cummings Veterinary Medical Center).

Catnip (Cummings Veterinary Medical Center).

LSU School of Veterinary Medicine.

Australian Veterinary Journal.

© 2020 WebMD, LLC. All rights reserved.


Get Fetch in your inbox

Veterinarian-approved information to keep your pet healthy and happy.

By clicking Subscribe, I agree to the WebMD Terms & Conditions & Privacy Policy and understand that I may opt out of WebMD subscriptions at any time.