What’s the Difference Between an Allergy and an Intolerance?

While an intolerance and an allergy both refer to a reaction from the body when a specific food source is ingested, they behave quite differently in the body.

It’s important to understand the difference between an allergy and intolerance, so you know how best to treat or manage your symptoms.

In this article, we will look at what a food allergy is, what a food intolerance is, as well as the specific differences between an allergy and an intolerance.

The main difference between an allergy and an intolerance

The differences between an allergy and an intolerance are mostly down to the body’s reaction, the symptoms, and the severity of those symptoms.

Some allergic reactions incur mild symptoms, while others can lead to a potentially life threatening response from the body. In contrast, an intolerance is very rarely life threatening, instead, you’re more likely to experience uncomfortable symptoms that last for a few hours.

Understanding the nuances of how they differ will help you identify whether you’re dealing with a food intolerance or allergic reaction symptoms. As the two have different treatment plans, it’s important to know which you’re experiencing.

For the main differences between an allergy and an intolerance, read the table, below:

To help you understand the nuances of each, we’ll explore both food intolerances and allergies in more detail, below.

What is a food intolerance?

A food intolerance is when your body struggles to digest certain foods or ingredients.

Otherwise known as a food sensitivity, some food intolerances are more common than others. For example, lactose, gluten, alcohol, histamine, and wheat are some of the most common products that people are intolerant to.

Symptoms of a food intolerance usually occur a few hours after ingesting the food and may linger for hours or days after, experiencing such symptoms as:

• Bloating.

• Stomach pain.

• Diarrhoea.

• Headache or migraine.

• Nausea.

As food intolerances involve your digestive system and not your immune system, they can be managed by avoiding the food or products containing problem ingredients. However, this isn’t the case for everyone.

With this in mind, it’s best to see your primary care clinician if your symptoms keep coming back despite efforts to keep the food out of your diet.

How do you test for a food intolerance?

As of yet, there are no scientifically approved ways to do a food intolerance test. However, you can conduct your own food sensitivity test at home.

One of the best ways to do this is to keep a food diary, as this will help you rule out foods you aren’t intolerant to and highlight the ones you may be experiencing a sensitivity to. To do this, try the following steps:

• Write down the foods you eat: Keep a note of when and what you eat and drink, including portion sizes and cooking methods, such as any oils or seasonings used.

• Take full notes: Be as detailed as you can. For example, don’t just write ‘sandwich’, instead write down ‘two slices of brown bread with butter, cheddar cheese, and honey roasted ham’. It can be beneficial to note down the brands of food here too, as different brands may use different ingredients.

• Track the symptoms: If you do experience any symptoms or discomfort, then jot down what they were and at what time you experienced them. This should help in seeing which foods are causing which symptoms.

• Keep the diary with you: It’s useful to have the food diary with you if you go out for a meal. As best as you can, make a detailed note of what you eat and drink even if it’s a dish you’ve had before elsewhere. For ease, this could be kept on your phone.

Once you’ve outlined the food you’re intolerant to, try your best to avoid eating it, as well as any dishes that may include it as an ingredient.

If your symptoms persist and you’re struggling to find the food you’re intolerant to, then it may be best to see your primary care clinician. They will be able to refer you to a dietician who can assist in finding the food causing the intolerance.  

What is a food allergy?

A food allergy is an immune system response to a certain food or ingredient after the body is exposed to the allergen. Essentially, the immune system mistakenly thinks the food is a threat. As a result, your body releases chemicals to get rid of it – which is what causes the allergic reaction.

While it’s more common for this reaction to occur once the food has been ingested, some may experience allergic reactions if the ingredient is airborne in an enclosed environment, such as an aeroplane.

Typically, the reaction will occur within a few seconds or minutes of ingesting the food. Though rare, some people do experience delayed reactions that can take up to 24 hours to show. Allergic reaction symptoms include:

• Tingling in the mouth.
• Itching skin.
• Rash or hives.
• Swelling of lips, face, or tongue.
• Stomach pain.
• Nasal congestion.
• Dizziness.

Common foods that can cause allergic reactions include shellfish, crustaceans, molluscs, peanuts, soybean, and sesame. However, it is possible to experience allergic reactions to a range of unique things from foods to specific ingredients.

If you know which food or foods you’re allergic to, then try to make every effort to avoid eating it. However, if you ingest the product by accident and experience a mild reaction, then taking an antihistamine may help relieve the symptoms.

When to seek medical attention for an allergy

While most cases of food allergies are mild and can be treated with antihistamines, there are occasions where it may be best to seek medical attention.

Some people experience severe allergic reactions to certain foods, medically known as anaphylaxis. If you have any of the following allergic reaction symptoms, then you may be dealing with anaphylaxis:

• Tightness of the airways. 

• Rapid pulse. 

• Shock.

• Swelling in the throat.

• Difficulty breathing.

• Low blood pressure.

If you or someone around you displays these symptoms, immediately call 999 or go to the nearest hospital for treatment.

How do you test for a food allergy?

There are different ways to test for an allergy, and your primary care clinician will be able to outline which is best for you, as well as refer you for an allergy test appointment.

Once you’ve spoken to your care provider, they will typically refer you for one of the following tests:

• A skin-prick test: A medical professional will place a drop of liquid onto your skin containing the potential allergen. They will then monitor for any signs of a response on your skin, such as inflammation, a rash, or hives.

• A blood test: A small blood sample will be taken via injection, this will then be sent off to a lab for testing. You will typically receive the results via your GP practice. If you’re allergic, then they will ask you to come in to discuss management plans.

How should potential allergens be to children?

Some foods carry a higher likelihood of causing an allergic reaction than others, so it’s important to be careful when introducing solid foods to a child’s diet.

The NHS recommends introducing potential allergens one at a time, so you’re able to monitor for reactions of each individually. Foods that commonly trigger an allergic reaction include:

• Cow’s milk.
• Eggs.
• Gluten, wheat, barley, and rye.
• Nuts and peanuts.
• Fish, particularly shellfish.
• Celery.
• Mustard.

When testing for an allergy or intolerance of peanuts or nuts, make sure they are finely ground or in the form of a nut butter, to make them easier to swallow. You only need a small amount to test with, about ¼ of a teaspoon is sufficient.

If you don’t notice any allergy symptoms from the child, you should maintain testing with a ¼ teaspoon level for a couple of weeks. You can gradually increase the measurements until you get to two teaspoons.

Introducing potential allergens to a baby can feel difficult. As such, it’s always best to consult your clinical team before commencing any trials yourself, to receive proper advice and support. Particularly in the case of a family history of certain allergies, eczema, asthma, or hay fever, as these can increase the chance of the baby having an allergy.

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Have a gluten intolerance and would like gluten free diet tips or you’re unsure when you should see a health professional? Head over to our Be Healthy hub for more useful health and wellbeing articles.

Medically reviewed by Cheryl Lythgoe in December 2024.