Biphasic Anaphylaxis – What you need to know

Biphasic reaction

What is a Biphasic Reaction?

A biphasic reaction is a two phase anaphylactic event. This means that after anaphylaxis is treated and the symptoms go away, they return without you being re-exposed to the allergen. The second reaction can be less severe, equal to or more severe than the first reaction. This makes them dangerous as some people may think that they are fully recovered and they may not have their adrenaline auto-injector as they will have used it to treat their first reaction. A second reaction can occur as little as 2 hours and as much as 72 hours after the first reaction. The average length between reactions is 6-10 hours.

How common are biphasic reactions?

Biphasic reactions occur in as many as 20% of anaphylactic reactions. Up to one third of patients who have had fatal or near fatal reactions have experienced a biphasic reaction.

Will I get a second reaction?

It’s impossible to tell for sure who will get a second reaction, but studies show that there are some factors that can increase the risk.

  • If the patient is a child between 6-9 years old.
  • If there is a delay in receiving emergency room treatment or a delay in administering adrenaline of greater than 90 minutes after the symptoms first appear.
  • If more than one dose of adrenaline is required.
  • If the respiratory symptoms require a dose of inhaled salbutamol.
  • If the blood pressure has a wide variation during the heart’s contractions.

What should I do?

If you suspect anaphylaxis you should always call an ambulance even if adrenaline is given. It’s important that the person is checked by doctors and monitored to ensure that the symptoms have disappeared. The doctors will usually keep the patient under observation for 4-6 hours after the initial symptoms are gone in order to monitor for a biphasic reaction.


It’s clear from above that a child whose symptoms are severe and whose treatment is delayed is at the highest risk for a biphasic reaction. This may mean that doctors will keep these children for longer observation periods, and children who have had milder symptoms and no delay in treatment may be allowed to go home sooner.

Always carry two adrenaline auto-injectors with you. Even if you only use one during anaphylaxis, you may need to use the second later in case of a biphasic reaction. Check out our range of twin adrenaline pen cases to make sure you always have them on you:

Holder for EpiPen auto-injectors – “Deluxe Black” Case for EpiPen auto-injectors - MediLifestyle Twin Case for EpiPen auto-injectors – Medbag Twin

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Disclaimer: The information provided is for informational purposes only and is not intended to be a substitute for professional medical advice, diagnosis or treatment. Allergy Lifestyle Limited (t/a) Allergy Lifestyle) uses reasonable endeavours to check the accuracy of information provided however no warranty is given that they are error-free.

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One thought on “Biphasic Anaphylaxis – What you need to know

  1. Robin Outz says:

    What happens if a child (12) has 3 reactions
    (1) immediately
    (2) 24 hr after more severe than (1st)
    (3) 72 hr after more severe yet than (2nd)
    Allergist told us today -after the (3) never heard of anything like it. Recommend a voice dr?what? Can you help us?

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