OTC sleep meds don't require a prescription, but your doctor should still know about any that you use regularly.

OTC sleep meds don't require a prescription, but your doctor should still know about any that you use regularly.(PRISCILLA DE CASTRO)When you're not sleeping, the quickest thing you can do is to head to the drugstore and grab a nonprescription sleep aid. Here's how to know what you're getting—and when you should use them.

Some OTC sleep aids are drug-free, containing herbs and the hormone melatonin. But most contain antihistamines such as diphenhydramine—medications used primarily to fight allergies by blocking histamine receptors in the brain. When it was discovered that they also caused drowsiness, antihistamines began being marketed for better sleep as well.

Common medication names
Here are some antihistamine products on the market.
• 40 Winks
• Benadryl
• Calm-Aid
• Compoz Nighttime Sleep Aid
• Nytol Caplet
• Simply Sleep
• Sleep MD
• Sleepinal
• Twilite
• Unisom Sleepgels Maximum Strength

More over-the-counter options

Pain relievers or cold and flu products touted as nighttime formulas often pair diphenhydramine with analgesics (acetaminophen or ibuprofen, for example), decongestants (such as pseudoephedrine), and/or cough suppressants (like dextromethorphan), and sometimes alcohol.

Risks and side effects
Just because these products are sold over the counter doesn't mean they don't have risks like any other drugs. According to the American Academy of Sleep Medicine, here are the most common side effects of antihistamines.
• Daytime sleepiness
• Dizziness
• Fatigue
• Headaches
• Reduced alertness
• Vomiting

Take caution with these medications, says Rajesh Balkrishnan, PhD, professor of pharmacy at Ohio State University College of Public Health in Columbus, and talk with your doctor about potential reactions to other substances you may be taking, including alcohol.

Tolerance and abuse
Antihistamine sleep aids are best for short-term use, because the body can quickly develop a tolerance; this is why you may find yourself drinking more and more Benadryl every night to put you to sleep, says David Rapoport, MD, medical director of the New York University Sleep Disorders Center. "It continues to work as an antihistamine, but the body adapts to the side effects—which include sleepiness," he explains.

And though it's rare, there is a small risk for abuse. "You can overdose on antihistamines," says Dr. Balkrishnan. "You'd have to take a lot to really put you in a coma—but even just one pill too many can leave you groggy in the morning."

Taking pills with additional ingredients—Tylenol or Advil PM, for example—gives you more to worry about. Because these pain medications have side effects of their own (especially with long-term use) and it's easy to overdose without realizing it, always let your doctor know if, and how often, you use these drugs.