Most reactions to food are intolerances, not allergies — and the difference matters for safety. A food allergy involves the immune system and can be life-threatening; a food intolerance is a digestion problem that makes you feel unwell but is not usually serious. Symptoms appear hours after eating with intolerance, and within minutes with allergy. This guide explains the difference, the red flags that need urgent care, and how intolerance is properly diagnosed. Reviewed 2026.
Food intolerance vs food allergy: the key difference
A food intolerance is when you have difficulty digesting certain foods or ingredients. It is not caused by the immune system overreacting, which means you cannot have a serious allergic reaction from it. Symptoms usually appear a few hours after eating and can last for hours or days.
A food allergy is different. It involves the immune system, and even small amounts of the food can trigger symptoms — sometimes within minutes. Symptoms include itchy skin, wheezing, or swelling of the lips, face, and eyes. In severe cases, an allergy can cause anaphylaxis, which is life-threatening.
Most physical reactions to food are caused by intolerance, not allergy — the two are often confused because some symptoms overlap.
Symptoms of a food intolerance
Common symptoms include:
- diarrhoea
- bloating
- farting (wind)
- tummy pain
There are many other possible symptoms, including headache, feeling tired or exhausted, feeling sick, constipation, joint pain, or rashes. Symptoms usually start a few hours after eating the food and can last for a few hours or days.
Common causes of food intolerance
An intolerance happens when your body cannot digest a certain food or ingredient. The most common is lactose intolerance — the body cannot digest lactose, a sugar mainly found in milk, cheese, and yoghurt. But you can be intolerant to any food or ingredient, including:
- gluten (wheat, rye, barley) — bread and pasta
- histamine — wine and aged cheese
- caffeine — coffee, tea, some fizzy drinks
- alcohol
- sulphites — cider, beer, wine
- salicylates — some fruits, vegetables, herbs, spices
- MSG — ripened fruits, cured meats, savoury foods
- benzoates — a preservative in soft drinks, juices, dressings, jams
Red flags: when to get urgent help
Call 999 (or your local emergency number) immediately if you or someone else has signs of a serious allergic reaction:
- lips, mouth, throat, or tongue suddenly become swollen
- breathing very fast or struggling to breathe — wheezing, choking, gasping
- throat feels tight or struggling to swallow
- skin, tongue, or lips turn blue, grey, or pale
- sudden confusion, drowsiness, or dizziness
- fainting and cannot be woken
- a child is limp, floppy, or not responding normally
These can be signs of anaphylaxis and need immediate hospital treatment. If you have had a severe allergic reaction before, carry your prescribed auto-injector and use it as directed.
How food intolerance is diagnosed
See a GP if symptoms keep coming back. If a GP thinks you have a food intolerance, you may be referred to a dietitian. Tests may include:
- An elimination diet — avoid the suspected food for a few weeks to see if symptoms improve, then reintroduce it to see if they return
- Blood tests
- A breath test — if lactose intolerance is suspected
- A food and symptoms diary — to help identify triggers
Important: home intolerance tests are not recommended. There is limited evidence they give accurate results, and they may suggest avoiding multiple foods, which can be harmful — especially for children.
What to do if you have a food intolerance
Try to avoid or reduce the food you are intolerant to. But do not cut foods out of your diet without advice from a GP or dietitian — you could miss out on vitamins and minerals that are especially important for a child's growth and development.
For lactose intolerance, many people tolerate small amounts of dairy, or choose lactose-free products. A dietitian can help you keep calcium intake while managing symptoms.
What does not help
- Home intolerance tests — limited evidence, often inaccurate, can lead to harmful food restriction.
- Cutting out whole food groups without advice — risk of nutrient deficiency.
- Assuming every reaction is an allergy — most are intolerances, but any breathing or swelling symptom must be treated as a possible allergy.
- Ignoring repeated symptoms — recurring symptoms deserve a proper diagnosis, not self-management.
Sources
- NHS — Food intolerance: nhs.uk/conditions/food-intolerance (page last reviewed 18 June 2026, accessed September 2026)
- NHS — Food allergy: nhs.uk/conditions/food-allergy
- NHS — Lactose intolerance: nhs.uk/conditions/lactose-intolerance
- Mayo Clinic — Food allergy vs. food intolerance: mayoclinic.org (February 2024)
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