ZzzQuil to Treat Insomnia and the Side Effects

Over-the-Counter Sleep Aid Contains Diphenhydramine

ZzzQuil is an over-the-counter sleep aid used to treat insomnia
ZzzQuil is an over-the-counter sleep aid used to treat insomnia. Getty Images

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 does ZzzQuil treat insomnia? What are some of the potential common and serious side effects from using ZzzQuil? Learn about ZzzQuil, and how it may help you to sleep better.

Uses of ZzzQuil

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's used to treat insomnia characterized by difficulty falling or staying asleep. It's meant to relieve occasional sleeplessness, and shouldn't be used for insomnia lasting more than 2 weeks.

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's advertised to be safer than other "PM" sleeping pills that may also contain tylenol, ibuprofen, or other ingredients. It should be taken when you can devote 8 hours to sleep to avoid morning side effects.

How Does ZzzQuil Work?

Since ZzzQuil’s main ingredient is diphenhydramine, it belongs to the group of medications called antihistamines. Antihistamines block the uptake of the chemical histamine into cells, resulting in a build-up of histamine in the spaces between nerve cells.

Histamine is a chemical messenger, or neurotransmitter, that can lead to drowsiness. Diphenhydramine and histamine work within the brain as well as other parts of the body. Diphenhydramine can also suppress cough, nausea, and uncontrolled movements called dyskinesias.

Who Should Not Use ZzzQuil?

It's recommended that you speak with your doctor 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 the elderly.

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 doctor 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 doctor or pharmacist before using it.

What Are Common Side Effects of ZzzQuil?

Some of the most common 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

What Are 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)

Necessary Safety Precautions When Taking ZzzQuil

As noted above, certain people are advised to use ZzzQuil with caution, or not at all. As it is an over-the-counter medication, the risks are believed to be slightly lower compared to other prescription medications used to treat insomnia. For pregnant women, harm to a developing fetus is possible but unlikely. In addition, ZzzQuil is probably safe during lactation, though caution is advised.

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.


"Diphenhydramine." Epocrates Rx Pro. Version 5.1.2, 2013. Epocrates, Inc. San Mateo, California.

ZzzQuil OTC Sleep-Aid. Last accessed: April 28, 2013.

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