Diphenhydramine (Benadryl®, Sominex®, and many others) is a medication available both over the counter and with a prescription. It comes in many different forms and is used for a variety of different purposes. Diphenhydramine is short for diphenhydramine HCl (HCl is the chemical symbol for hydrochloride).
Diphenhydramine is part of a class of drugs called antihistamines. Specifically, it is an H1 receptor antagonist. This means that it blocks a specific type of histamine receptor in the body. Since allergic reactions are partly caused by the release of histamine from certain cells in the body, diphenhydramine can help relieve or prevent allergy symptoms.
The medication also blocks acetylcholine receptors, an action which produces some of the bothersome side effects of the medication (such as dry mouth or difficulty urinating) but also makes the drug useful for some uses (such as for Parkinson's disease or for relieving a runny nose due to the common cold).
Like many antihistamines, diphenhydramine also works as a sedative. This action has made the medication useful as a sleep aid but also limits the usefulness of the medication for other uses (since drowsiness can be a bothersome side effect).
(Click Diphenhydramine for more information on what diphenhydramine HCl is used for, to learn about the potential side effects of this medicine, and to find out what forms and strengths are available for this drug.)