Allergies and Intolerances: Symptoms, Causes and Treatment

Reactions to everyday substances like food, pollen, or pet dander are common — but not all reactions are the same. Understanding whether you have an allergy or an intolerance helps you get the right treatment and avoid unnecessary worry or dietary restriction.

What is an allergy?

An allergy occurs when your immune system reacts abnormally to a substance that's typically harmless to most people. These substances, known as allergens, can be found in everyday items like pollen, animal dander, or certain foods such as nuts, fruit, or shellfish.

With an allergy, even a small amount of the trigger can cause a reaction, and reactions can range from mild to severe or life-threatening (Patient.info, 2026).

What is a food intolerance?

A food intolerance (sometimes called a food sensitivity) is different from an allergy. It happens when your body struggles to digest a particular food — often because it's missing an enzyme needed to break it down, as with lactose intolerance — rather than your immune system overreacting (NHS, 2026).

Food intolerances mainly cause digestive symptoms such as bloating, gas, stomach pain, or diarrhoea, and symptoms can last from a few hours to a few days (NHS, 2026). Unlike allergies, intolerances are not caused by the immune system, so they cannot lead to a serious allergic reaction such as anaphylaxis (NHS, 2026).

Common examples of food intolerance include:

  • Lactose intolerance (difficulty digesting milk and dairy products).
  • Non-coeliac gluten intolerance.
  • Fructose intolerance.

Allergy vs intolerance: what's the difference?

 

Food allergy

Food intolerance

What's affected

Immune system

Digestive system

How much triggers it

Even a tiny amount

Usually depends on quantity eaten

Onset

Rapid — often within minutes to two hours

Can take hours to appear

Typical symptoms

Hives, swelling, wheezing, breathing difficulty

Bloating, gas, stomach pain, diarrhoea

Can it be life-threatening?

Yes — risk of anaphylaxis

No

Typical symptoms

Allergy symptoms vary from person to person. In most cases they cause mild irritation, such as sneezing, a runny or blocked nose, itchy eyes, coughing, or an itchy rash. In some cases, allergic reactions can be more severe, leading to breathing difficulties or anaphylaxis.

Intolerance symptoms are usually digestive and include bloating, gas, cramping, diarrhoea, or fatigue, and are not typically life-threatening (NHS, 2026).

man lying on a sofa holding a tissue to her face, feeling unwell

When to see a health professional

Allergy and intolerance symptoms can sometimes overlap with other conditions, so it's not always easy to tell them apart. If you're unsure, a GP or Advanced Clinical Practitioner (ACP) can help.

  • For mild allergies or suspected intolerances, our clinicians can offer advice, and, where appropriate, refer you to a dietitian to help identify trigger foods (NHS, 2026).
  • For more severe reactions, or if the cause is unclear, we can refer you to an allergy specialist for further tests.

Seek urgent medical help if you or someone else has swelling of the lips, tongue or throat, difficulty breathing, or sudden confusion or dizziness after eating — these can be signs of anaphylaxis (NHS, 2026).

Frequently asked questions

No — they're caused by different mechanisms in the body, so an intolerance doesn't develop into an allergy. However, it's important to get a proper diagnosis, as symptoms can overlap (Mayo Clinic, 2026).

A GP may suggest an elimination diet, where you avoid a suspected food for a few weeks before reintroducing it to see if symptoms return, sometimes alongside a food and symptom diary. The NHS advises against relying on unregulated over-the-counter intolerance tests, which aren't recommended (NHS, 2026).

 Call 999 or go to A&E immediately if you have swelling of the face or throat, difficulty breathing, or dizziness/confusion after exposure to a possible allergen. If you carry an adrenaline auto-injector (such as an EpiPen), use it as directed and seek emergency help straight away.

References

(NHS, 2026) https://www.nhs.uk/conditions/food-intolerance/

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