Biphasic Anaphylaxis: Why a Second Reaction Can Happen Hours Later

Anaphylaxis is a severe allergic reaction that requires immediate treatment with epinephrine. But did you know that some people experience a second wave of symptoms hours after the initial reaction? This is called biphasic anaphylaxis, and understanding it can be lifesaving.

What Is Biphasic Anaphylaxis?

Biphasic anaphylaxis occurs when an initial allergic reaction resolves, only for a second reaction to appear hours later—sometimes even without further exposure to the allergen.

When Does the Second Reaction Occur?

✔️ Usually 1 to 12 hours after the first reaction.
✔️ Can occur up to 72 hours later in rare cases.
✔️ Symptoms may be milder or just as severe as the initial reaction.

Symptoms of Biphasic Anaphylaxis

The second reaction can mimic the first and may include:

• Hives, swelling, or itching
• Wheezing, shortness of breath
• Drop in blood pressure, dizziness, fainting
• Nausea, vomiting, diarrhea
• Throat tightness or difficulty swallowing

Who Is at Risk?

Some people are more prone to biphasic reactions, including those who:

• Had a severe initial anaphylactic reaction.
• Delayed epinephrine treatment.
• Required multiple doses of epinephrine.
• Have a history of asthma or multiple allergies.

How to Prevent and Manage Biphasic Anaphylaxis

✔️ Always go to the ER after using epinephrine, even if symptoms improve.
✔️ Monitor for at least 4–6 hours after the first reaction.
✔️ Carry 2 epinephrine auto-injectors in case of a second reaction.
✔️ Take antihistamines and steroids as prescribed to help prevent recurrence.
✔️ Follow up with an allergist to identify triggers and create an emergency action plan.

FAQ: Understanding Biphasic Reactions

Q: Does everyone with anaphylaxis experience a second reaction?
A: No, but up to 20% of anaphylaxis cases may have a biphasic component.

Q: Can biphasic anaphylaxis be prevented?
A: Early epinephrine use may reduce the risk, but monitoring is still essential.

Q: Should I take prednisone to prevent a second reaction?
A: Some doctors prescribe steroids, but their effectiveness in preventing biphasic reactions is debated.

A Personal Story

My patient, Jake, had a peanut allergy and used his epinephrine auto-injector after accidental exposure. He felt better within minutes but was sent to the ER for observation. Four hours later, he developed new hives and breathing trouble—a classic biphasic reaction. Because he was still in the hospital, he received a second epinephrine dose immediately, preventing a more severe episode.

The Bottom Line

Biphasic anaphylaxis is unpredictable, but being prepared can save lives. If you or a loved one has a history of severe allergic reactions, always carry two epinephrine auto-injectors and seek emergency care after any anaphylactic event.

What are you waiting for?  If you or your child need help controlling allergies or if you think you need testing or treatment or that you might be a good candidate for allergy shots, Dr. Wendt and her team staff at Relieve Allergy, Asthma & Hives would love to help. Call for testing or treatment today!

Call 480-500-1902 or request an appointment at https://relieveallergyaz.com/contact now!  Begin your allergy testing journey with Dr. Wendt at Relieve Allergy in Scottsdale, Arizona—It’s time to take care of yourself!

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Relieve Allergy Asthma & Hives is located near Kierland Commons, 21803 N. Scottsdale Road Ste. 200, and has convenient evening and morning hours to accommodate your schedule. Dr. Wendt is also available for telemedicine appointments as appropriate. Most insurance plans are accepted.

DISCLAIMER: This blog is not meant to substitute for medical care by an Allergist-Immunologist nor is it intended to provide medical advice or imply a Physician-Patient relationship with its readers or their family. It is intended to provide guidance, direction, and moral support from someone experienced in testing and treating allergies. This is also not meant to be a “do-it-yourself” manual or a “how to” overcome your allergies. Some of the issues mentioned, whether allergic or look-a-like, can be dangerous or if left unchecked, deadly. Please develop a relationship with a Physician you trust to help you with diagnosis, advice, questions, and treatment regarding your and your family’s medical conditions.