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. When symptoms return in this way, this is known as biphasic anaphylaxis.
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 would be for your treating doctor to decide. On very rare occasions, a biphasic reaction has been known to occur as long as 72 hours after the initial reaction.
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.
More about anaphylaxis – click here.
The text of this article has been peer reviewed by Prof John Warner, Professor of Paediatrics Imperial College London; early years theme lead for CLAHRC NW London; Hon Professor University of Cape Town. He has no conflicts of interest to declare.
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Published: Dec 2016
Review date: Dec 2019