Diphenhydramine: Uses, How It Works, and Safety Information

Overview

Diphenhydramine is an antihistamine medication best known for treating allergy symptoms, but it is also widely used as an over-the-counter sleep aid. It is the active ingredient in brand names like Benadryl, ZzzQuil, Sominex, and many generic sleep products.

While diphenhydramine can be effective for occasional sleeplessness, it is not intended for long-term use as a sleep aid. The medication causes drowsiness as a side effect of its antihistamine properties, rather than working with the body’s natural sleep mechanisms.

How Diphenhydramine Works

Diphenhydramine belongs to a class of medications called first-generation antihistamines. It works by:

Blocking histamine receptors (H1 receptors) in the brain and body
Crossing the blood-brain barrier, which causes sedation
Reducing wakefulness through central nervous system depression
Blocking other receptors (anticholinergic effects) that contribute to sedation

Unlike melatonin, which works with the body’s natural sleep-wake cycle, diphenhydramine induces drowsiness through its sedating properties.

Common Uses

Diphenhydramine is used for:

Sleep-Related:

Occasional insomnia or difficulty falling asleep
Short-term sleep disturbances due to travel or schedule changes
Temporary sleep problems

Allergy-Related:

Allergic reactions and hay fever symptoms
Itching from hives or insect bites
Runny nose and sneezing

Other Uses:

Motion sickness prevention
Nausea and vomiting
Cough suppression (in some formulations)

When used as a sleep aid, diphenhydramine is intended for short-term, occasional use only.

Forms Available

Diphenhydramine is available in several forms:

Tablets and capsules: Typically containing 25 mg or 50 mg
Liquid formulations: Syrups and solutions
Chewable tablets: For those who prefer not to swallow pills
Combination products: Often paired with pain relievers for nighttime cold relief

Sleep aid products typically contain diphenhydramine hydrochloride, the same active ingredient found in allergy medications.

Who Should Avoid Diphenhydramine

Certain individuals should not use diphenhydramine or should consult a healthcare provider before use:

People with narrow-angle glaucoma
Those with enlarged prostate or difficulty urinating
People with asthma, chronic obstructive pulmonary disease, or other breathing conditions
Individuals with cardiovascular disease or high blood pressure
Those taking monoamine oxidase inhibitors (MAOIs)
People with hyperthyroidism or increased thyroid activity
Older adults (due to increased risk of cognitive side effects and falls)

Effectiveness as a Sleep Aid

Diphenhydramine can help people fall asleep faster when used occasionally. However:

Limitations:

Tolerance develops quickly (often within a few days to weeks)
It does not improve overall sleep quality or sleep architecture
It causes hangover effects and next-day drowsiness
It does not address underlying causes of insomnia
Long-term use is not recommended

Research suggests that diphenhydramine’s effectiveness as a sleep aid decreases with repeated use as the body becomes accustomed to it.

Next-Day Effects

A significant consideration with diphenhydramine is its long duration of action:

Effects can last 4-6 hours or longer
Next-day drowsiness and grogginess are common
Impaired coordination and reaction time may persist into the next day
Cognitive performance may be reduced

These effects can interfere with driving, operating machinery, or performing tasks requiring alertness.

Safety During Pregnancy and Breastfeeding

Diphenhydramine is generally considered safe for occasional use during pregnancy when needed, but pregnant individuals should consult a healthcare provider before use.

Diphenhydramine passes into breast milk in small amounts. Nursing mothers should discuss use with a healthcare provider, as it may cause drowsiness in infants or reduce milk supply.

Concerns in Older Adults

Diphenhydramine is listed as a potentially inappropriate medication for older adults due to:

Increased risk of confusion and cognitive impairment
Higher likelihood of falls and injuries
Anticholinergic effects (dry mouth, constipation, urinary retention)
Potential worsening of dementia symptoms

Many geriatric experts recommend avoiding diphenhydramine as a sleep aid in people over 65.

Duration of Use

For sleep problems:

Use only occasionally, not every night
Do not use for more than 2 weeks without consulting a healthcare provider
If sleep problems persist, seek medical evaluation to identify underlying causes

Chronic insomnia requires evaluation and treatment of root causes rather than ongoing use of sedating antihistamines.

Dependence and Withdrawal

Diphenhydramine is not physically addictive in the way that prescription sleep medications can be. However:

Psychological dependence can develop
People may feel unable to sleep without it
Stopping after prolonged use may temporarily worsen sleep

Sources

This information is based on FDA-approved prescribing information, peer-reviewed medical literature, and clinical practice guidelines.