You've given your patient IM adrenaline, they're breathing normally and feeling well again. You'd like to discharge them with an Epipen but there's a little voice at the back of your head: "What if they have a biphasic reaction at home and they can't take the pen in time?" Prior studies have found biphasic reaction rates as high as 20%, and there are reports of reactions happening as long as 4 days after the initial anaphylaxis. Studies so far have tended to be underpowered, retrospective and with widely differing definitions of 'allergy' and 'biphasic reaction' (we're not really concerned about a slight rash appearing on day 2, after all).

Anesthesiology, November 2018

Annals of Emergency Medicine, June 2014