How are antihistamines used as sleep aids?

The old antihistamines, like Benadryl, have a very drowsy side effect that makes them useful as sleep aids. It is this side effect that people use to help get to sleep using an antihistamine. Some of the new ones do not have such a significant drowsy side effect.
Dr. Darria Long Gillespie, MD
Emergency Medicine
Older types of antihistamine drugs make many people feel sleepy. This is an unwanted side effect for people trying to treat allergy symptoms. But 2 antihistamines are widely used as sleep aids. They can be used by themselves or combined with pain relievers, such as acetaminophen and ibuprofen, in "night-time" or "PM" formulas. The drugs are diphenhydramine (for example, Nytol, Sominex, Unisom) and doxylamine (for example, Unisom). They are available without a prescription. 

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