Over-the-counter sleep aid medications that contain diphenhydramine, such as Excedrin PM, Nytol and Tylenol PM, are recommended by WebMD. Diphenhydramine, like other sleep aids, works by causing drowsiness. However, the most common use of diphenhydramine is as an allergy medication.
Diphenhydramine is most effective in treating mild cases of insomnia, states WebMD, and it is not effective as a long-term solution. Side effects include drowsiness in the morning, difficulty urinating, confusion and delirium. Side effects often occur in users over the age of 65. Younger people should avoid taking diphenhydramine for more than two weeks due to increases in tolerance.