Diphenhydramine HCl is an antihistamine. You can get it over the counter as Benadryl or many various generic offerings in 25mg doses to treat allergy symptoms. The main side effect, however, is drowsiness.
But have no fear, for some it’s a side effect, but for others, that’s exactly what they are looking for. So, interestingly enough, you can also get it as a nighttime sleep aid, just down the aisle. Same dosage, same availability in generic form: just a different color pill.
Even more interesting is that one randomized, double-blind, crossover study has shown that tolerance to the sedative effect of the drug after just three days was indistinguishable from a placebo. So that’s good news for the allergy sufferers who don’t want to be sleepy in the daytime, but not so good news for the people that can’t sleep and attempt to use it a few days in a row.