Dredged up for Journal Club from The Year 2000 (that was the future, once).
You might scoff at this article because it enrolls a grand total of 25 participants with acute urticaria, ten of which receive diphenhydramine and fifteen receive famotidine. You might be more impressed to know that this is pretty reflective of the evidence we have regarding H2-blockers in the treatment of urticaria. Another study from 1993 compares diphenhydramine, famotidine, and cromolyn sodium – and only enrolls 20!
It is mildly amusing to see them report there is no significant difference between the groups when they don’t have the power to detect any. Regardless, our H2 blockers provide some relief, it’s likely additive, and they’re inexpensive and safe.
The “definitive” study at present supporting our H1 + H2 blocker for acute urticaria or allergic reaction in the emergency department enrolls 91, and it shows diphenhydramine + cimetidine is superior to diphenhydramine alone.
http://www.ncbi.nlm.nih.gov/pubmed/10844490
http://www.ncbi.nlm.nih.gov/pubmed/8329794
http://www.ncbi.nlm.nih.gov/pubmed/11054200