Anaphylaxis prophylaxis? Maybe...
This retrospective study of almost 2000 patients with allergic reactions concluded that receipt of an H1-antihistamine (H1a) reduced the number of patients who progressed to develop anaphylaxis, 1.9% vs. 3.4% in those who did not receive an H1a. The NNT was 45, assuming this was a true association. But it's hard to be sure, given that the groups were not equal: more in the H1a group received epinephrine and steroids prior to progressing to anaphylaxis. They statistically adjusted for this possible confounder and others and found the beneficial effect of H1a still held. They also performed a propensity score adjusted model which showed the beneficial effect of H1a. This study adds weak evidence that antihistamines are helpful in allergic reactions and may decrease progression to anaphylaxis. But take these conclusions with a healthy dose of skepticism.
It makes sense to treat an allergic reaction with an antihistamine. This study suggests that treating patients with an allergic reaction with a H1-antagonist may decrease the chance of disease progression to anaphylaxis, though the level of evidence is weak. First 10 EM has an excellent analysis.
H1-antihistamines Reduce Progression to Anaphylaxis Among Emergency Department Patients With Allergic Reactions. Acad Emerg Med. 2017 Jun;24(6):733-741. doi: 10.1111/acem.13147.
Peer reviewed by Thomas Davis.