When a person is allergic to onions, their immune system reacts as if the onions are a dangerous substance. Onion allergy symptoms include vomiting, itching, swelling, and difficulty breathing.

As onions are a commonly used ingredient in food, they can be difficult to avoid, so it is important for people to be aware if they are allergic.

Onions are part of the allium family, a plant group that includes onions, garlic, leeks, scallions, chives, and shallots.

This article will explore the symptoms that nutritionists associate with an onion allergy, as well as how to diagnose and treat it.

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Symptoms of an onion allergy may include nausea, headache, and itching.

If food comes into contact with onions, it may trigger an allergic reaction.

If a person is allergic to onions, symptoms can appear up to 2 hours after ingestion.

Symptoms of an onion allergy may include:

  • nausea
  • vomiting
  • diarrhea
  • headache
  • itching
  • hives or rash
  • swelling of the face, lips, or throat
  • trouble breathing
  • stomach pain
  • coughing or wheezing
  • tingling sensation in the mouth
  • fainting or dizziness
  • anaphylaxis, although extremely rare

The main difference between a food allergy and food sensitivity is the system of the body that is involved.

A food allergy involves the immune system, whereas food sensitivity or intolerance involves the digestive system.

When a person is allergic to onions, their body treats the onion as an invader. The immune system releases antibodies called immunoglobulin E (IgE), which trigger allergic reactions.

According to the American Academy of Allergy, Asthma, and Immunology (AAAAI), every time a person is exposed to the food, an allergic reaction can occur.

Food allergy symptoms can be severe and trigger anaphylactic shock. If a person is experiencing anaphylactic shock, they need emergency medical attention.

Symptoms of anaphylactic shock may include:

  • hives
  • tightening of the throat
  • vomiting
  • diarrhea
  • rapid heartbeat
  • fainting
  • dizziness

Food sensitivity

A person can experience food sensitivity when the body is unable to digest certain foods.

This can lead to digestive problems such as nausea, bloating, diarrhea, or gas.

People who are sensitive to onions may experience digestive problems after consuming them. However, in some cases, these symptoms may not appear until hours later.

A food sensitivity can cause uncomfortable symptoms but is not life threatening.

Symptoms of food sensitivity may correlate with the amount of offending food a person eats. For example, a tiny amount of onion may trigger no symptoms or mild symptoms, whereas eating a larger amount of onion may cause more severe symptoms.

A doctor or allergist may diagnose a food allergy with a blood test, skin test, or both.

A blood test will look for amounts of certain immune antibodies.

During an allergy skin test, a healthcare provider places a tiny amount of liquid food extract on the skin. They will then use a small instrument to prick the skin.

If, after 15–20 minutes, a raised bump appears, it is likely that the person is sensitive to that particular food.

Although there is no treatment for food allergies, people can manage their allergies in the following ways:

People who are allergic to onions should avoid them altogether.

The Food and Drug Administration (FDA) do not require food manufacturers to list onions as an allergen. However, the FDA require that manufacturers list the ingredients in their products, so looking for onions in the ingredients list may be helpful.

Cross contamination or inhalation of the food may also trigger an allergic reaction in some people.

This means a person with an onion allergy may have to avoid:

  • eating foods that could have onion-derived ingredients, for example, onion powder
  • any food that could have come into contact with onions or onion products
  • being around people while they are chopping or cooking onions
  • selecting meals at restaurants that could have been on shared equipment or dishes with onions

The American College of Allergy, Asthma, and Immunology (ACAAI) recommend that people with food allergies should wear a medical alert bracelet that lists their allergy triggers.

The ACAAI also recommend that people at risk carry an epinephrine auto-injector device, such as an EpiPen.

A person should carry two epinephrine auto-injector devices in case of a severe allergic reaction.

Antihistamines may also help. These can alleviate symptoms of mild allergic reactions, such as itching.

If a person is allergic to onions, they may find that other alliums trigger a reaction also.

According to an article in the World Journal of Methodology, doctors call this cross-reactivity. The IgE antibodies may recognize and bind themselves to similar allergen molecules, causing an immune response and triggering an allergic reaction.

A doctor may identify these allergies during testing.

Food allergy symptoms can be life threatening.

If a person notices any allergic symptoms, such as swelling, trouble breathing, itching, and hives, they should immediately seek emergency care.

If a person thinks they have trouble digesting onions, they should discuss this with a doctor.

There is no known cure for an onion allergy.

Keeping a food diary and eliminating certain foods can help a person identify food intolerances.

If a person consumes an onion, and they start to feel unwell, they may be allergic to onion.

As onion allergies can be life threatening, it is vital that a person speaks with a doctor if they experience any symptoms of an allergic reaction.

If a person has any symptoms of an anaphylactic shock, they should seek emergency medical help.

To prevent experiencing an allergic reaction, a person can avoid onions and be careful when buying food items.

Antihistamines can help with mild symptoms, but it is essential that a person has epinephrine auto-injector devices on them in case they experience severe allergic reactions.