skip to main content
How does ZzzQuil Work? Article


In need of a good night’s sleep so you can wake up feeling refreshed? Look no further than ZzzQuil™ Sleep-Aid Products. But how does ZzzQuil work—and how should this medication be taken?

Although ZzzQuil is brought to you by the makers of Vicks® NyQuil®, it’s not for colds. It’s not for pain. It’s just for sleep.

ZzzQuil contains diphenhydramine HCI, which is a big word for a small compound that helps you sleep. To help you stay awake and alert, your body produces histamine, a naturally occurring substance that interacts with receptors in your brain to promote wakefulness. Diphenhydramine acts as an antihistamine, which blocks your brain’s alertness to make you feel the opposite: drowsy.1

Studies have shown that diphenhydramine relieves occasional sleeplessness when used as directed, by reducing the time it takes for you to fall asleep so you feel rested in the morning.2

The other great news about diphenhydramine? For more than 30 years, diphenhydramine has been approved by the FDA as an OTC (over-the-counter) medicine to treat occasional sleeplessness.3 Plus, diphenhydramine is non-habit-forming.

ZzzQuil Liquid and ZzzQuil LiquiCaps® Sleep-Aid Products contain diphenhydramine HCI so you can get a good night’s sleep—and wake up refreshed.

Brought to you by:
ZzzQuil OTC Sleep-Aid Products and the The National Sleep Foundation

Emanuel MB. Histamine and the antiallergic antihistamines: a history of their discoveries. Clinical and Experimental Allergy. 1999; 29:1–11.
U.S. Code of Federal Regulation. 21 CFR 338—Nighttime sleep aid drug products for over-the-counter human use. 14 February 1989.
U.S. Food and Drug Administration. Diphenhydramine; Marketing status as a nighttime sleep-aid drug product for over-the-counter human use; Notice of enforcement policy. Federal Register. 1982;47(79):17741. Available at: Accessed 15 March 2012.