Food intolerance vs food allergy: what's the difference?

The plain-English difference between a food allergy and a food intolerance, why it matters for reading labels, and when symptoms are worth getting properly diagnosed.

People often use the words allergy and intolerance as if they mean the same thing, but they describe two different things happening in the body. Understanding the difference helps you take symptoms seriously where it counts, read labels in the right way, and know when it is worth getting a proper diagnosis. This is general information, not a substitute for medical advice.

Two different processes in the body

A food allergy is an immune system response. The body mistakes a harmless food protein for a threat and reacts against it, sometimes within minutes. Allergic reactions can range from mild to severe, and in some people a small amount of the food is enough to trigger one. At the serious end, an allergy can cause a reaction that affects breathing and needs emergency treatment.

A food intolerance is a digestive response. Here the immune system is not involved. Instead, the body struggles to break down or handle a particular food, which leads to symptoms such as bloating, tummy pain, wind or feeling generally unwell, often building up over hours rather than striking suddenly. It can make you feel rotten, but it works through a completely different mechanism from an allergy.

The same food, such as milk, can be involved in both: a milk allergy is an immune reaction to milk protein, while lactose intolerance is the digestive system struggling with the sugar in milk. Same food, two very different situations.

Why the difference matters for labels

Knowing which one you have changes how strictly you need to read labels. For a true food allergy, even trace amounts can matter, which is why UK law requires the 14 major allergens to be emphasised in the ingredients list, and why the voluntary “may contain” line exists to flag possible cross-contamination. For someone managing a serious allergy, those bold words and warnings are essential reading every single time.

For an intolerance, the picture is often about quantity. Some people can tolerate a small amount of a food without trouble but feel unwell with more. The label still matters, but the decision is frequently about how much rather than absolutely none. Treating an intolerance with the same caution as a severe allergy can mean cutting out foods you did not need to, while treating a real allergy as “just an intolerance” can be genuinely dangerous. Knowing which you have keeps your label-reading proportionate and safe.

When to get it properly diagnosed

If your symptoms are recurring, severe, or affecting your daily life, it is worth seeing your GP rather than self-diagnosing. Cutting major foods out of your diet on a hunch can leave gaps in your nutrition and can also mask what is really going on. A proper assessment can tell you whether you are dealing with an allergy, an intolerance, or something else entirely, and that clarity is genuinely useful.

Be cautious of unofficial “intolerance tests” sold directly to the public, which are not always reliable. Your GP is the right starting point, and they can refer you for proper testing if needed. If you ever have signs of a severe reaction, such as difficulty breathing or swelling of the face or lips, treat it as an emergency and call 999.

Whichever you are managing, knowing exactly what is in your food is the foundation. Read the label carefully every time, and use The Food Allergy Lab to check the hidden names a food can travel under.

Written and reviewed by the Food Allergy Lab editorial team.