Diphenhydramine Side Effects

Overview

Diphenhydramine causes drowsiness by design when used as a sleep aid, but it also produces other side effects due to its antihistamine and anticholinergic properties. Many of these effects can be bothersome and may persist into the next day, affecting daily functioning and quality of life.

Understanding these side effects is important when deciding whether diphenhydramine is appropriate for occasional sleep problems.

Common Side Effects

The most frequently reported side effects of diphenhydramine include:

Next-Day Effects:

Drowsiness and grogginess (“hangover effect”)
Difficulty concentrating or thinking clearly
Impaired coordination and reaction time
Reduced alertness and performance
Feeling mentally foggy or “out of it”

Anticholinergic Effects:

Dry mouth and throat
Dry eyes or blurred vision
Constipation
Difficulty urinating (especially in men with enlarged prostate)
Increased heart rate

Other Common Effects:

Dizziness or lightheadedness
Upset stomach or nausea
Thickening of mucus in airways
Headache
Loss of appetite

These effects are common and can occur even with proper dosing. The severity varies among individuals.

Next-Day Impairment

A significant concern with diphenhydramine as a sleep aid is its long duration of action:

Residual effects can include:

Difficulty waking up in the morning
Feeling groggy or drowsy throughout the following day
Impaired driving ability (comparable to alcohol impairment in some studies)
Reduced work or school performance
Slower reaction times
Memory and concentration problems

These effects can last 6-8 hours or longer after taking the medication, often extending well into the next day even after a full night’s sleep.

Less Common Side Effects

Some people may experience:

Confusion or disorientation (especially in older adults)
Restlessness or paradoxical excitation (more common in children and some adults)
Nightmares or vivid dreams
Tremor or shakiness
Nervousness or anxiety
Ringing in the ears
Loss of coordination
Weakness or fatigue

Serious Side Effects (Rare)

While uncommon, some serious side effects require immediate medical attention:

Severe Allergic Reactions:

Rash, hives, or itching
Swelling of the face, lips, tongue, or throat
Difficulty breathing or swallowing
Severe dizziness

Cardiovascular Effects:

Irregular or rapid heartbeat
Chest pain or tightness
Significant increase in heart rate

Neurological Effects:

Severe confusion or hallucinations
Seizures (rare but possible, especially in overdose)
Difficulty with balance or walking

Urinary Effects:

Inability to urinate or severe difficulty urinating
Painful urination

If any of these serious effects occur, stop using diphenhydramine and seek immediate medical attention.

Anticholinergic Side Effects

Diphenhydramine blocks acetylcholine, a neurotransmitter involved in many body functions. This anticholinergic action causes numerous side effects:

Short-term anticholinergic effects:

Dry mouth, eyes, and throat
Blurred vision or difficulty focusing
Constipation
Urinary retention or difficulty starting urination
Increased heart rate
Confusion (especially in older adults)

Concern with long-term use: Some research suggests that chronic use of anticholinergic medications may be associated with increased risk of cognitive decline and dementia in older adults. While occasional use is unlikely to pose this risk, it is another reason to avoid using diphenhydramine as a long-term sleep solution.

Risk Factors for Side Effects

Certain factors increase the likelihood or severity of side effects:

Older Adults (65 and Older)

Seniors are more vulnerable to diphenhydramine’s effects:

Higher risk of confusion, disorientation, and delirium
Increased likelihood of falls and injuries
Greater sensitivity to anticholinergic effects
More pronounced cognitive impairment
Longer duration of effects

Many geriatric experts consider diphenhydramine inappropriate for older adults due to these risks.

Enlarged Prostate

Men with benign prostatic hyperplasia (BPH) may experience:

Worsened difficulty urinating
Urinary retention
Increased urinary frequency or urgency

Glaucoma

People with narrow-angle glaucoma should avoid diphenhydramine, as it can worsen this condition and potentially trigger an acute glaucoma attack.

Breathing Conditions

Those with asthma, chronic obstructive pulmonary disease, or other respiratory conditions may experience:

Thickening of lung secretions
Difficulty clearing mucus
Worsened breathing difficulty

Higher Doses

Taking more than the recommended dose significantly increases the risk and severity of all side effects without improving sleep effectiveness.

Drug Interactions That Increase Side Effect Risk

Central Nervous System Depressants:

Combining diphenhydramine with other sedating substances dramatically increases drowsiness, impairment, and risk:

Alcohol (significantly increases sedation and risk of accidents)
Benzodiazepines and sleeping pills
Opioid pain medications
Muscle relaxants
Certain antidepressants

Other Anticholinergic Medications:

Taking diphenhydramine with other anticholinergic drugs compounds these effects:

Tricyclic antidepressants
Medications for overactive bladder
Some antipsychotic medications
Certain medications for Parkinson’s disease

MAOIs:

Monoamine oxidase inhibitors can intensify and prolong diphenhydramine’s anticholinergic effects.

Tolerance and Reduced Effectiveness

With repeated use:

The sedating effect diminishes rapidly (often within days to weeks)
Side effects may persist even as sleep benefits decrease
People may be tempted to increase doses, worsening side effects
The medication becomes less effective while maintaining its risks

This tolerance pattern is a key reason diphenhydramine is not suitable for chronic sleep problems.

Paradoxical Reactions

Some people, especially children and occasionally adults, experience paradoxical excitation instead of sedation:

Increased energy and restlessness
Agitation or hyperactivity
Difficulty falling asleep despite taking a sleep aid
Irritability

If this occurs, discontinue diphenhydramine and consider alternative sleep aids.

Cognitive and Memory Effects

Diphenhydramine can impair:

Short-term memory formation
Attention and concentration
Processing speed
Learning ability
Decision-making

These effects are most pronounced during the medication’s active period but can extend into the next day.

Safety During Pregnancy and Breastfeeding

Pregnancy: Diphenhydramine is generally considered safe for occasional use during pregnancy, though drowsiness may be more pronounced. Pregnant individuals should consult a healthcare provider before use.

Breastfeeding: Diphenhydramine passes into breast milk and may cause drowsiness in nursing infants. It may also reduce milk supply in some people. Nursing mothers should discuss use with a healthcare provider.

Minimizing Side Effects

To reduce the likelihood and severity of side effects:

Use the lowest effective dose (start with 25 mg rather than 50 mg)
Take only occasionally, not nightly
Allow 7-8 hours for sleep before needing to be alert
Avoid alcohol and other sedating substances
Do not drive or operate machinery until you know how you respond
Stay hydrated to reduce dry mouth
Discontinue use if side effects are bothersome

When to Stop Using Diphenhydramine

Discontinue diphenhydramine and contact a healthcare provider if you experience:

Severe next-day impairment affecting daily functioning
Confusion, hallucinations, or unusual behavior
Difficulty urinating or inability to urinate
Rapid or irregular heartbeat
Severe dizziness or fainting
Signs of allergic reaction
Any side effect that concerns you or significantly affects your quality of life

Comparison with Other Sleep Aids

Compared to Melatonin: Diphenhydramine generally causes:

More side effects overall
Stronger next-day hangover effects
More anticholinergic effects
Greater cognitive impairment

Compared to Prescription Sleep Medications: Diphenhydramine is available without a prescription but:

May cause comparable next-day drowsiness
Has more anticholinergic side effects
Is not designed specifically for sleep
Develops tolerance more quickly

Sources

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