How ZzzQuil Sleep Aid Treats Insomnia

ZzzQuil, an over-the-counter sleep aid that contains the generic drug diphenhydramine, is used to treat people experiencing difficulty falling or staying asleep. How well does ZzzQuil work to treat insomnia? What are some of the potential common and serious side effects from using ZzzQuil? Learn about ZzzQuil, what it does to you, and how it may help you to sleep better.

What Is ZzzQuil?
Verywell / Cindy Chung


ZzzQuil is sold over-the-counter, meaning that you don't need a prescription to purchase it. Sold as a liquid or a liquid-filled capsule, ZzzQuil is a sleep aid that is used to treat insomnia characterized by difficulty falling or staying asleep.

ZzzQuil is meant to relieve occasional sleeplessness, and shouldn't be used for insomnia lasting more than two weeks. If insomnia persists, cognitive behavioral therapy for insomnia (CBTI) is recommended.

ZzzQuil's active ingredient, diphenhydramine, is most often used to provide relief from allergies; sleepiness is a side effect of this component. But since diphenhydramine is the medication's only active ingredient, it is advertised to be safer than other "PM" sleeping pills that may also contain Tylenol (acetaminophen), ibuprofen, or other ingredients. It should be taken when you can devote eight hours to sleep to avoid morning side effects.

How It Works

Since ZzzQuil’s main ingredient is diphenhydramine, it belongs to the group of medications called antihistamines. Antihistamines block the action of the chemical histamine, a signal within the central nervous system that promotes wakefulness. As a result, antihistamine medications can lead to drowsiness. Diphenhydramine and histamine work within the brain as well as other parts of the body, causing the potential for side effects. Diphenhydramine can also suppress a cough or nausea and may lead to uncontrolled movements called dyskinesias.

Diphenhydramine, the main ingredient of ZzzQuil, is only modestly effective in improving sleep, according to research. Studies suggest that people fall asleep on average about 8 minutes faster compared to placebo. The total amount of sleep improves by an average of about 12 minutes.

The American Academy of Sleep Medicine discourages the use of the medication as a sleep aid because the benefits are approximately equal to the potential for harm.

Who Can Use It

Speak with your healthcare provider if you're concerned about how ZzzQuil may affect your health or interact with your other medications. In general, it should not be used in children under 12, and it should be used with caution in older people.

Some of the conditions that may require additional caution when considering the use of ZzzQuil include:

  • Glaucoma
  • Hyperthyroidism
  • Heart disease
  • High blood pressure
  • Asthma
  • Chronic obstructive pulmonary disease (COPD)
  • Pneumonia
  • Peptic ulcer disease
  • Prostate enlargement
  • Obstruction of the bowel or bladder

If you have any of these conditions, you should speak with your healthcare provider before starting to use ZzzQuil. As this medication has the potential to interact with other drugs, especially those that impact the brain, review your medications with your healthcare provider or pharmacist before using it. You can use our Healthcare Provider Discussion Guide below to start that conversation.

Insomnia Healthcare Provider Discussion Guide

Get our printable guide for your next healthcare provider's appointment to help you ask the right questions.

Doctor Discussion Guide Woman

Side Effects

Morning "hangover" effects may occur with the use of the medication, especially if inadequate time is spent in bed after taking it; at higher doses; with the use of other medications; or in the presence of certain medical conditions, including untreated sleep disorders like sleep apnea. Some of the side effects that may commonly occur with the use of ZzzQuil include:

  • Confusion (delirium)
  • Clumsiness
  • Drowsiness
  • Dizziness
  • Increased pressure within the eye
  • Headache
  • Stomach upset
  • Dry mouth or nose
  • Hyperactivity
  • Constipation
  • Thick lung secretions
  • Difficulty urinating
  • Low blood pressure
  • Blurry or double vision
  • Rapid or irregular heart rate
  • Light sensitivity
  • Sweating
  • Erectile dysfunction

Potential Serious Reactions

As with the use of any drug, there are also risks of serious side effects with the use of ZzzQuil. These occur more rarely compared to the common side effects listed above. When using ZzzQuil, these may include:

  • Anaphylaxis (severe allergic reaction including difficulty breathing)
  • Low blood cell counts (including anemia)
  • Abnormal heart rhythms
  • Seizures
  • Toxic psychosis (characterized by confusion and abnormal behaviors)
  • Acute labyrinthitis (inner ear problems)


As noted above, certain people are advised to use ZzzQuil with caution, or not at all. As it is an over-the-counter medication, there may be an impression that the overall risks may be slightly lower compared to other prescription medications used to treat insomnia. This may not be the case, however, and review of additional options should occur with your healthcare provider.

Taking more than the prescribed dose, or using it with alcohol, may lead to overdose and serious health consequences. If you experience any difficulties with the use of ZzzQuil, you should be in close contact with your primary health provider.

For pregnant women, harm to a developing fetus is possible but unlikely. In addition, ZzzQuil is probably safe during lactation, though caution is advised.

Given the limited effectiveness of this medication, it may be desirable to pursue other treatment options for insomnia.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Sateia MJ, Buysse DJ, Krystal AD, Neubauer DN, Heald JL. Clinical practice guideline for the pharmacologic treatment of chronic insomnia in adults: An American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(2):307-349. doi:10.5664/jcsm.6470

  2. National Institutes of Health. ZzzQuil nighttime sleep aid - diphenhydramine hydrochloride liquid.

  3. Food and Drug Administration. Diphenhydramine hydrochloride injection, USP label.

  4. NIH Toxicology Data Network. Diphenhydramine.

  5. Gilboa SM, Ailes EC, Rai RP, Anderson JA, Honein MA. Antihistamines and birth defects: a systematic review of the literature. Expert Opin Drug Saf. 2014;13(12):1667-98. doi:10.1517/14740338.2014.970164

  6. Drugs and Lactation Database (LactMed). Diphenhydramine. Bethesda (MD): National Library of Medicine (US).

By Brandon Peters, MD
Brandon Peters, MD, is a board-certified neurologist and sleep medicine specialist.