Biphasic anaphylaxis involves a second anaphylactic reaction that happens after the first has been successfully treated. This may occur up to 72 hours after the initial reaction.

Anaphylaxis is a potentially life threatening allergic reaction that is known to have a rapid and unpredictable onset.

Symptoms can start minutes after exposure to an allergen, which is any substance that causes you to have an allergic response.

Biphasic anaphylaxis is a recurrence of anaphylaxis after appropriate treatment. It happens without any additional exposure to the allergen. Think of it as anaphylaxis part two.

A single anaphylactic reaction is called a uniphasic reaction. Biphasic anaphylaxis strikes when you’ve survived the initial attack and all seems well. The second attack can occur up to 72 hours after the first. Symptoms of a second reaction commonly peak within 8 to 11 hours after the first reaction.

Due to the risk of biphasic anaphylaxis, your doctor may want you to stay in the hospital after the initial attack so that they can monitor your condition.

The symptoms of biphasic anaphylaxis are the same as anaphylaxis symptoms, but they may differ in severity.

Symptoms of this second phase are typically mild or moderate, but there’s no guarantee that the second event won’t become life threatening. Every episode of anaphylaxis requires immediate medical attention.

An estimated 1.6% to 5.1% of people worldwide may experience anaphylaxis at some point in their lifetime. The true incidence of biphasic anaphylaxis is unknown, but it may occur in up to 20% of anaphylaxis cases.

After exposure to an allergen, a series of potentially alarming events takes place throughout your body:

  • Your skin develops redness or other color changes, becomes itchy, and may swell or develop hives.
  • Your airways begin to close, and breathing becomes increasingly difficult.
  • Your tongue and mouth swell.
  • Your blood pressure drops.
  • You may feel pain in your abdomen.
  • You may have diarrhea.
  • You may experience vomiting.
  • You may lose consciousness.
  • You may experience shock.

Both uniphasic and biphasic anaphylaxis are medical emergencies and require immediate treatment, preferably in the hospital emergency room. Without treatment, anaphylaxis can be fatal.

The cause of biphasic anaphylaxis is not fully understood. There’s no accurate way to identify all people who are more likely to experience this medical event, but the risk factors include:

  • a history of anaphylaxis
  • an allergy without a known cause
  • symptoms such as diarrhea and wheezing

Any allergen can cause anaphylaxis, but some are more likely to trigger it, such as:

Epinephrine is the primary medication used to treat anaphylaxis. It’s fast and effective at opening your airways and reducing other symptoms.

Epinephrine is available as an auto-injector. The person experiencing the attack or someone who is with them can inject the medication if medical help isn’t nearby. The brand most people are familiar with is EpiPen.

If your doctor decides that you need to carry an auto-injector, they’ll give you a prescription for one and show you how it works. The device is easy to use:

  1. To prepare the auto-injector, flip open the cap of the clear carrier tube and slide the injector out of the tube.
  2. Hold the auto-injector with the orange tip pointing down. Keep EpiPen’s trademarked phrase in mind: “Blue to the sky, orange to the thigh.”
  3. Remove the blue safety cap by pulling it straight up, but do not bend or twist it. It’s best to hold the auto-injector in one hand and use your other hand to remove the cap.
  4. Place the orange tip against the middle of your outer thigh, at a right angle to your thigh. Swing out and push in, holding firmly for 3 seconds.
  5. Remove the auto-injector and massage the area for 10 seconds.

If the blue safety release is raised or if the auto-injector doesn’t readily slide out of the carrier tube, you should not use it. Instead, contact your doctor and the manufacturer.

Even if you feel better after the injection, it’s important to seek medical help. If you have a history of severe allergic reactions, you always need to carry an epinephrine auto-injector and know how to use it.

It’s critical to identify the cause of an anaphylactic reaction so you can avoid that allergen in the future.

In some cases, your doctor might recommend immunotherapy, or allergy shots, which can reduce your body’s response to the allergen.

If your doctor prescribes an epinephrine auto-injector, carry it with you. Show family members and others close to you how to use it too.

Call 911 immediately if you or someone you’re with is experiencing an attack of anaphylaxis. Your goal is to get professional medical care as quickly as possible.

If you’re with someone who is having an anaphylactic reaction, take the following steps:

  1. Ask if they have an epinephrine auto-injector.
  2. If they have one and they cannot inject the medication themselves, inject it for them.
  3. Help them get comfortable and elevate their legs, if possible.
  4. If needed, perform CPR.

A biphasic anaphylactic reaction involves a recurrence of symptoms after the initial symptoms of anaphylaxis have been treated and subsided. A second reaction can occur up to 72 hours after the first.

This is one reason it’s important to go to the hospital after you experience an anaphylactic reaction. Even if you’ve used epinephrine and your symptoms have gone away, they could come back. You may need to stay at the hospital for a few hours to be monitored for a biphasic reaction.