A food allergy is the immune system overreacting to an ingredient, usually a protein, while a food intolerance is a digestive problem with no immune reaction involved. In cats and dogs the symptoms overlap so much that vets usually start the same way for both: a strict food trial followed by reintroduction.
The difference still matters, because it shapes how strict you need to be in the long run and what you should watch out for. Here’s what each term means in plain language.
What is a food allergy in dogs and cats?
A food allergy happens when the immune system treats an ingredient in food as a threat. In pets, the trigger is usually a protein, and often one they’ve been eating for months or years. The PDSA’s guides to food allergies in dogs and food allergy in cats describe the typical signs:
- Skin: itching, often around the ears, paws, face and belly in dogs, and the head and neck in cats.
- Coat: fur loss from licking and overgrooming.
- Ears: recurring ear problems.
- Tummy: vomiting, diarrhoea or wind.
Because the immune system is involved, even small amounts of the trigger can be enough to set symptoms off again. Severe, sudden reactions are rare in pets, but they can happen.
What is a food intolerance in pets?
A food intolerance doesn’t involve the immune system. The gut simply struggles to cope with a particular food or ingredient. The classic example is milk: many adult cats and dogs don’t digest lactose well, so dairy can cause loose stools or wind.
Intolerance signs are mostly digestive: soft stools, diarrhoea, wind, vomiting and tummy discomfort. They are often related to how much was eaten, so a small amount may cause no trouble while a larger amount does.
What about upsets that are neither?
Plenty of tummy trouble has nothing to do with allergy or intolerance:
- Scavenging: bin raids, dropped picnic food and things found on walks.
- Sudden diet changes: switching food too quickly.
- Rich or fatty food: roast leftovers, skin and gravy.
- Poisoning: some human foods, such as chocolate, grapes, raisins, onions, garlic and the sweetener xylitol, are toxic to pets.
- Infections and parasites, which cause many of the same signs.
How do the symptoms compare?
| Food allergy | Food intolerance | Environmental allergy | |
|---|---|---|---|
| What’s happening | Immune reaction to a food ingredient, usually a protein | The gut can’t handle a food well; no immune reaction | Immune reaction to pollen, dust mites, moulds or similar |
| Main signs | Itchy skin and ears, often with tummy signs | Mostly tummy signs: diarrhoea, wind, vomiting | Itchy skin, paws and ears |
| Pattern | All year round, while the food is eaten | Often linked to amount and timing of a meal | Often seasonal or linked to places |
| How vets investigate | Strict food trial, then reintroduction | Strict food trial, then reintroduction | Ruling out other causes; sometimes skin or blood tests |
| Long term | Strict avoidance of the trigger | Often avoiding or limiting the food | Managing exposure and symptoms with your vet |
Why do vets treat both the same way at first?
You can’t reliably tell an allergy from an intolerance by looking at the symptoms, and the PDSA notes that blood tests aren’t accurate for food allergies in pets. So the practical question becomes “is food involved, and which food?”
That’s what a food trial answers. Your pet eats one vet-recommended diet and nothing else for several weeks, then old foods go back one at a time. Our step-by-step guides to the dog elimination diet and the cat elimination diet explain how to run one.
The result guides what happens next. If a single protein brings symptoms back quickly and strongly, your vet may advise strict lifelong avoidance. If a food causes milder, dose-related tummy trouble, cutting it down may be enough.
Can a pet have more than one problem?
Yes, and it’s common. The PDSA notes that cats are often allergic to more than one thing. A dog may have an environmental allergy that flares in summer and a food reaction that grumbles all year. Fleas can add to either.
That’s why record-keeping helps so much. Our guides to dog food intolerance symptoms and cat food intolerance symptoms include simple diary layouts. If you’d like to understand the same distinction in people, see our guide to food intolerance vs allergy in humans.
When to speak to a vet
- Facial swelling, hives, collapse or breathing difficulty: go to an emergency vet straight away. Intolerance testing is not an allergy test and is no substitute for veterinary investigation.
- Your pet may have eaten something poisonous: call your vet immediately, without waiting for signs.
- Vomiting or diarrhoea with blood, lethargy, or a cat that stops eating.
Where hair sample testing fits
Whether your pet has an allergy, an intolerance or both, the food trial still needs a starting point: which proteins to avoid and which foods to test first. That’s where testing can help.
Our Pet Essential 500 intolerance test tests 500 food and environmental items, plus metals, vitamins and minerals and selected additives, from a small sample of your pet’s hair, brushed or snipped and bagged separately. You order online and the instructions arrive by email straight away.
The report groups items into high, medium and low response. It doesn’t tell you whether your pet has an allergy or an intolerance; use it with your vet to plan a food trial, which is where that answer comes from.
Frequently asked questions
Are food allergies common in dogs and cats?
Food reactions do happen in both, but fleas and environmental allergies are more common causes of itching. That's why vets usually rule those out before starting a food trial.
Can a pet grow out of a food allergy or intolerance?
Some pets can tolerate a food again later, especially with milder, dose-related intolerances, but true food allergies usually need long-term avoidance. Never test a food that caused a severe reaction without your vet's advice.
Is milk bad for cats and dogs?
Many adult cats and dogs don't digest lactose well, so milk and cream often cause loose stools or wind. It's a common example of a food intolerance. Fresh water is all they need to drink.
Do hypoallergenic pet foods help with intolerance too?
They can, because they remove or break down common proteins and are usually simpler recipes. But not every food labelled 'sensitive' or 'hypoallergenic' is suitable for a strict trial, so ask your vet which diet to use.
This guide is general wellness information, not medical advice. Our test uses bioresonance, a complementary (CAM) method; it does not diagnose allergies, coeliac disease or any medical condition. Speak to a GP if you have severe or ongoing symptoms.
