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    Allergies vs intolerances

    On this page
    1. What’s the difference between an allergy and an intolerance?
    2. Allergic reaction causes
    3. Intolerance causes
    4. Are my symptoms caused by a food allergy or a food intolerance?
    5. Symptoms of a non-IgE-mediated food allergy
    6. When should I speak to a doctor about my symptoms?
    7. In an emergency
    8. When it’s not an emergency
    9. Treatments for allergies and intolerances
    10. Get allergy treatment from Online Doctor

    Allergy and intolerance signs

    Having an allergy or an intolerance is really common. In the UK, allergic conditions like hay fever, asthma, and eczema are widespread amongst children and adults, and food intolerances are on the rise.

    If you’re concerned that you might have developed an allergy or an intolerance, and you’re confused about the difference between the two, read on for a simple guide.

    What’s the difference between an allergy and an intolerance?

    In medicine, the label “intolerance” refers to a sensitivity to a substance you’ve consumed. An allergy, meanwhile, can be brought on by any kind of exposure. You might have an allergic reaction after eating, breathing in, or touching the substance you’re allergic to.

    Another important difference between the two is that allergies can lead to anaphylaxis and be life-threatening. Intolerances are not life-threatening, although they can cause unpleasant symptoms.

    Allergic reaction causes

    An allergic reaction happens because of the immune system. In people with allergies, the immune system reacts to certain substances like pollen and dust as though they’re harmful, and releases chemicals like histamine to protect the body. Whenever the body is exposed to that same allergen, the immune response leads to allergic symptoms like swelling and itching. 

    Common substances that can produce an allergic reaction include foods like shellfish, nuts, milk, and eggs, as well as pollen, mould, dust, and animal dander.

    Intolerance causes

    An intolerance has nothing to do with the immune system. It happens when your body has sensitivity to and/or difficulty digesting a certain type of food, drink, or medicine. This causes symptoms like stomach pain, bloating, wind, and diarrhoea, as well as skin rashes and itching.

    It’s fairly common to have an intolerance to lactose or wheat.

    Are my symptoms caused by a food allergy or a food intolerance?

    If you’re having unpleasant symptoms after eating, you might be unsure about the exact cause. Food allergies and food intolerances can both cause symptoms like stomach pain, diarrhoea, and rashes.

    However, there are usually some key differences. This checklist may be able to help you narrow it down:

    • Did your symptoms come on very quickly or gradually? An allergy usually causes a reaction quickly i.e. within seconds or minutes. Symptoms of an intolerance tend to come on more gradually, over a few hours. 
    • Did you eat a small or a large amount of the food? An allergic reaction can happen after consuming tiny amounts of the food you’re allergic to. If you have a food intolerance, you’ll usually need to eat a substantial amount. 
    • Did you have an allergic reaction? If you’ve had an allergic reaction to something you’ve eaten or drunk, you may experience tingling or itching in the mouth, a raised itchy rash, swelling of the mouth and throat, difficulty swallowing, and wheezing or shortness of breath. An intolerance might cause rashes and itching, but it won’t cause the other symptoms.

    Symptoms of a non-IgE-mediated food allergy

    It’s important to note that not all food allergies will adhere to the checklist above. If you have something called a “non-IgE-mediated” food allergy, you probably won’t have the classic allergic reaction. Your symptoms may look more like the symptoms of a food intolerance.

    With a non-IgE-mediate allergy, symptoms typically take much longer to develop, and include

    • Red and itchy skin 
    • Eczema flare-ups 
    • Vomiting 
    • Diarrhoea 
    • Stomach cramps 
    • Constipation 
    • In babies, colic

    It’s possible to experience these symptoms alongside a traditional allergic reaction. This is known as a “mixed reaction” and can happen in children who have a milk allergy.

    When should I speak to a doctor about my symptoms?

    In an emergency

    Symptoms brought on by an allergic reaction can cause anaphylaxis. This is a medical emergency and requires immediate help – you should call 999 if you notice the symptoms of anaphylaxis

    • Difficulty breathing 
    • Wheezing 
    • Trouble swallowing and speaking 
    • Feeling dizzy 
    • Fast heartbeat 
    • Clammy skin 
    • Confusion 
    • Losing consciousness

    This may be accompanied by classic allergic reaction symptoms e.g. a rash.

    When it’s not an emergency

    It’s a good idea to talk your GP (or a specialist like a dietician) if any of the following apply: 

    • You think you or your child might have a food allergy
    • You’re regularly experiencing symptoms like diarrhoea, bloating, stomach pain, and skin rashes, and you aren’t sure of the cause
    • You want to try an elimination diet, where you cut out foods and reintroduce them to see if this affects your symptoms

    If you’re only having irregular, mild symptoms, you may not need to see a doctor yet. Instead, try keeping a food diary. Write down what you’ve eaten, any symptoms you’ve had, and when symptoms have occurred. If your symptoms persist you can speak to your GP or a diet specialist about making changes to your diet.

    Treatments for allergies and intolerances

    If your GP thinks you have an allergy or an intolerance, they may want to refer you to an allergy clinic for testing, or to a dietician. 

    If your GP isn’t sure what’s causing your symptoms they may need to do some tests. There are several conditions that can cause the kinds of symptoms associated with a food allergy or a food intolerance, including coeliac disease, irritable bowel syndrome, and inflammatory bowel disease.

    Get allergy treatment from Online Doctor

    At Online Doctor we can prescribe treatment for people with allergies like hay fever. Visit our secure online allergy clinic to find out more. 


    Authors and editors

    • Written by

      Dr Sameer Sanghvi
      GMC number: 7085078
      Date published: 14th April 2021

    • Reviewed and updated by

      Dr Neel Patel
      GMC number: 7085151
      Date reviewed: 29th September 2023

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