Anaphylaxis is a potentially life-threatening allergic reaction. The first symptoms usually occur very rapidly and most cases are relieved by prompt treatment but occasionally the symptoms return. This usually happens within hours of the initial reaction but on rare occasions they can be more delayed. This is known as biphasic anaphylaxis.
Therefore anyone who suffers a severe allergic reaction (anaphylaxis) needs to be observed in hospital after they recover in case a biphasic reaction occurs. The length of the observation period is for your treating doctor to decide. When you are discharged from hospital make sure you have been provided with at least one adrenaline auto injector, which is the treatment for anaphylaxis.
We are aware of cases where people who have suffered anaphylaxis have decided not to dial 999 but have waited for the symptoms to resolve on their own, which can happen. These people were unaware that a biphasic reaction could occur. We must stress that anyone suffering anaphylaxis needs emergency medical treatment as soon as possible after the first symptoms occur.
Research shows that it is difficult to predict whether biphasic anaphylaxis will occur after the initial reaction. It seems more likely to happen if a drop in blood pressure (hypotension) is a symptom of the initial reaction. This will show as feeling light-headed, faint, extremely tired, or becoming unconscious.
More about anaphylaxis – click here.
More about adrenaline – click here.
The text of this article has been peer reviewed by Prof John Warner, Emeritus Professor of Paediatrics, National Heart and Lung Institute, Imperial College London; Hon Professor University of Cape Town. He has no conflicts of interest to declare.
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Published: Jan 2020
Review date: Jan 2023