The Facts

Anaphylaxis is a medical emergency. Though rare, this serious type of allergic reaction occurs after a person is exposed to an allergen (a substance they are allergic to), such as certain foods, medications, or insect stings.

The body's immune system quickly reacts, causing swelling of the face and throat, difficulty breathing, dangerously low blood pressure, and leaky blood vessels. These effects can lead to shock. If not treated quickly, anaphylaxis can be fatal.

About 1% to 2% of Canadians are at risk of an anaphylactic reaction. Avoiding the allergen and knowing what to do if you are exposed are the keys to managing the problem.


Just about any substance can bring on an anaphylactic reaction. But anaphylaxis is most commonly caused by:

  • allergy shots
  • blood products
  • foods such as peanuts, tree nuts, shellfish, eggs, fish, sesame, soy, milk, and wheat
  • insect bites or stings
  • latex (a type of natural rubber found in certain medical supplies)
  • medications such as antibiotics, vaccines, anti-inflammatories and painkillers, and dyes used for CT or MRI scans

Sometimes, the cause for the anaphylaxis is never found. The medical term for this is idiopathic anaphylaxis.

When a person with an anaphylactic allergy is exposed to an allergen, their immune system goes into overdrive. The substances the body produces (e.g., histamine) are intended to protect the body from a foreign invader, but they overreact, causing the throat to swell up and the blood vessels to leak fluid. This leads to the symptoms of anaphylaxis and can be life-threatening.

People with asthma, seasonal allergies, or eczema are at a higher risk of anaphylaxis. Although anaphylaxis rarely occurs, it can happen at any age. Race and geographic location do not affect the risk of anaphylaxis.

Symptoms and Complications

The most common symptoms of anaphylaxis include:

  • dangerously low blood pressure, dizziness, sweating, and fainting
  • hives
  • nausea, cramping, diarrhea, or vomiting
  • swelling of the face and throat
  • wheezing or difficulty breathing

Sometimes, anaphylaxis can cause other symptoms as well. These include:

  • chest pain
  • flushing
  • headache
  • itching
  • seizures

If you experience these symptoms, seek immediate medical attention. Anaphylaxis can be fatal if not treated. The usual causes of death are cardiovascular collapse, which means that the heart cannot supply the body with the oxygen it needs; or laryngeal edema, which means that the throat swells up and prevents the person from breathing in enough oxygen to survive.

Sometimes, the symptoms of an anaphylactic reaction can return anywhere from 1 to 72 hours after the first attack. This is called biphasic anaphylaxis and happens in about 20% of cases. The cause of biphasic anaphylaxis remains unknown, as does a method of predicting the return of symptoms.

Your doctor may wish to keep you for a period of observation after your first attack has been treated. If your anaphylaxis symptoms come back, seek immediate medical attention.

Making the Diagnosis

The diagnosis of anaphylaxis is based on symptoms. The doctor will evaluate your symptoms to rule out other conditions. In general, anaphylactic reactions are easier to recognize while they are happening and more difficult to diagnose after the attack. If anaphylaxis was not diagnosed during the attack, it can also be diagnosed after the fact using lab tests and a description of your symptoms.

The cause of an anaphylactic reaction is often clear - you may experience a reaction shortly after being stung by an insect, eating a certain food, or taking a certain medication. If the cause is not clear, additional tests may be done to determine the cause. These include keeping a diary, skin testing, and blood tests. Because anaphylaxis is a medical emergency, tests to find out the cause of the reaction will be delayed until after the reaction is treated.

Treatment and Prevention

Anaphylaxis requires immediate medical attention. If you are having a reaction, use your epinephrine* kit (e.g., EpiPen®, Twinject®) as instructed by your doctor and call 9-1-1. Lie down with your legs elevated to combat the effects of low blood pressure that can occur with the reaction.

Once medical help arrives, health professionals will check your vital signs (pulse, breathing rate, temperature, and blood pressure) and see whether you need oxygen or more epinephrine. Epinephrine is given to counteract the effects of your body's reaction to the allergen. Oxygen is given (using a tube or mask) when you are having trouble getting enough oxygen through breathing.

More than one dose of epinephrine may be needed to deal with the reaction. You may also receive an inhaled medication such as salbutamol to open the airways, antihistamines or steroids to "tone down" your body's reaction to the allergen, and medications to increase your blood pressure to safe levels (anaphylaxis can cause dangerously low blood pressure).

After your reaction is treated, your doctor will ask you to remain at the hospital for a period of time. This is important, as the reaction may come back within 1 to 72 hours (biphasic anaphylaxis).

Anaphylaxis can be treated. But the best way of dealing with anaphylaxis is to prevent an anaphylactic reaction from happening in the first place. Here are a few things you can do to protect yourself:

  • Know your allergies. Talk to your doctor about things that may give you a reaction, how to recognize the symptoms of an anaphylactic reaction, and what to do if they occur.
  • Have desensitization therapy (allergy shots) if you have an insect venom allergy.
  • Follow all treatment for asthma carefully to reduce your risk of a life-threatening anaphylactic reaction.
  • Be sure that all health professionals involved in your care are aware of your allergies and ensure that this information is kept on your file.
  • Wear a MedicAlert® bracelet (or similar identification) identifying your allergies.
  • Always have an epinephrine kit with you. Ideally, have 2 doses of epinephrine available. Be sure you know how to use your epinephrine injection and check regularly to make sure it is not expired.
  • Take all reasonable steps to avoid the things you are allergic to.


*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.