How to keep a food reaction diary to find triggers

Updated 29 October 2026 · By IngrediLens

You had hives after lunch, or your skin flared after a new moisturiser, but you are not sure what caused it. A reaction diary turns vague memories into something your doctor can work with. It is simple to keep and is one of the most useful things you can bring to an allergy appointment.

Important: if someone has signs of a severe reaction (difficulty breathing, swelling of the throat or tongue, feeling faint), use their adrenaline auto-injector if they have one and call emergency services. The diary can wait.

Why a diary helps

Doctors diagnose food allergy mainly from your history: what happened, how quickly and after what. Tests such as skin prick tests or specific IgE blood tests are then used to confirm or rule out suspects. A good diary helps your doctor:

• spot patterns (the same ingredient in several products, reactions only after exercise)
• tell the difference between immediate allergic reactions, which usually come within minutes to about two hours, and delayed symptoms that may point to an intolerance or another cause
• choose which tests make sense and avoid unnecessary ones

What to write down

For every reaction, record:

Cofactors such as exercise, alcohol, some painkillers (NSAIDs), illness, stress and lack of sleep can make a reaction more likely or more severe. That is why they are worth noting.

For skin reactions to cosmetics, note the product, where you applied it, how long after the rash started, and keep the packaging. See reading cosmetic ingredient lists.

Also note what you tolerated

It is just as useful to note foods you ate without problems, especially if they contain an ingredient you suspected. If you ate wheat bread every day without symptoms, wheat is less likely to be the cause of a reaction to a sandwich. The filling becomes the main suspect.

Common mistakes

• Cutting out lots of foods on your own. Restrictive diets can cause nutritional problems, especially in children, and can make diagnosis harder. Coeliac disease tests, for example, need you to keep eating gluten until testing is done.
• Reintroducing a food that caused a severe reaction "to test it". Food challenges should only be done under medical supervision.
• Trusting home test kits. The NHS and allergy organisations warn that tests such as IgG food intolerance tests, hair analysis, kinesiology and "vega" testing are not reliable for diagnosing allergies.
• Recording only the main ingredient. "Pasta" is not enough. The sauce may contain milk, egg, nuts or mustard. Our guides list hidden milk, egg and nut names.
• Forgetting "may contain". A reaction to a product that only had a precautionary warning is important information for your allergist. See what "may contain" means.

Taking the diary to your appointment

Bring the diary, photos of labels, and any products or packaging that caused a reaction. Write down your questions beforehand, for example:

• Is this likely to be an allergy, an intolerance or something else?
• Which tests do I need?
• Do I need an adrenaline auto-injector and an action plan?
• Should I avoid "may contain" products?

How IngrediLens helps

IngrediLens Premium includes a reaction diary. Log what happened after a product (symptoms, how bad and when) and link the scan of its label. The diary then looks for ingredients that keep turning up in products you reacted to, compared with the other products you scanned. That gives you something concrete to discuss with your doctor or allergist. You can also create a doctor summary PDF to take to your appointment. The diary stays on your device. It shows patterns in your own notes, not a diagnosis, so talk to a doctor before changing what you eat or use.