Hirschowitz_1983_J.Pharmacol.Exp.Ther_226_171

We quantitated the gastric secretory anticholinergic effect of diphenhydramine (Benadryl) against graded doses of bethanechol, a cholinergic agonist, which was used to stimulate acid and pepsin secretion. Anticholinergic potency of diphenyhydramine was 10,000 times less than that reported for atropine and was 1,000 times less than the reported potency of diphenhydramine antagonism of histamine H-1 effects.