Diphenhydramine is an antihistamine used primarily to treat allergy symptoms, relieve coughs from colds, prevent motion sickness, and help with short-term sleep problems. You probably know it by the brand name Benadryl, though it appears in dozens of over-the-counter products including sleep aids like ZzzQuil and combination cold medicines.
Allergy and Cold Relief
The most common reason people reach for diphenhydramine is allergies. It relieves itchy, watery eyes, sneezing, runny nose, and hives by blocking histamine, the chemical your immune system releases during an allergic reaction. Histamine is what makes blood vessels leak fluid into surrounding tissue, causes skin to itch and swell, and triggers mucus production in your nose and airways. Diphenhydramine blocks the receptor that histamine latches onto, effectively shutting down that entire chain of symptoms.
It also suppresses the cough reflex in your brain, which is why it shows up in many cold and cough formulas. This makes it useful for the kind of dry, irritating cough that comes with upper respiratory infections, though it won’t do much for a deep chest cough caused by mucus buildup.
Short-Term Sleep Aid
Diphenhydramine crosses easily from the bloodstream into the brain, which is why it causes drowsiness. That side effect became a selling point: it’s the active ingredient in most over-the-counter sleep aids. In clinical trials, a 50 mg dose significantly reduced the time it took people with mild to moderate insomnia to fall asleep. One study found that after five consecutive nights, people taking diphenhydramine reported falling asleep faster and sleeping longer than those on a placebo, and even outperformed a common prescription sleep medication on self-reported sleep duration.
The catch is that this benefit is strictly short-term. An expert consensus panel unanimously agreed that diphenhydramine should not be used for chronic insomnia lasting more than three months, and recommended a maximum treatment window of about four weeks. Your body builds tolerance to the drowsiness effect relatively quickly, meaning it stops working as well after regular use. It can also impair daytime performance the morning after, leaving you groggy or mentally sluggish.
Motion Sickness Prevention
Diphenhydramine is effective at preventing nausea, dizziness, and vomiting from motion sickness. For this purpose, you need to take it before you start traveling, not once symptoms have already begun. Taking it 30 minutes before a car ride, boat trip, or flight gives it time to start working. The motion sickness benefit comes partly from its ability to block acetylcholine, a brain chemical involved in the nausea signals that your inner ear sends when it detects conflicting motion cues.
How It Works in the Body
Diphenhydramine does several things at once, which explains both its versatility and its long list of side effects. Its primary action is blocking histamine receptors found throughout the body, on everything from airway muscles and blood vessels to immune cells and brain neurons. By blocking these receptors, it reverses the swelling, itching, flushing, and airway constriction that histamine causes.
Because it’s a first-generation antihistamine (meaning it was developed before newer versions like cetirizine and loratadine), it passes freely into the brain. Newer antihistamines were specifically designed not to do this, which is why they don’t cause drowsiness. In the brain, diphenhydramine blocks histamine receptors involved in wakefulness, producing sedation. It also blocks acetylcholine receptors, which are structurally similar to histamine receptors. This acetylcholine-blocking activity is responsible for many of its side effects but also makes it useful for reducing tremors in Parkinson’s disease.
Common Side Effects
Most side effects trace back to diphenhydramine’s action on acetylcholine, the chemical messenger involved in muscle control, digestion, and gland secretion. Blocking it produces a recognizable cluster of symptoms:
- Drowsiness and impaired coordination, the most noticeable effect for most people
- Dry mouth and dry eyes, because acetylcholine normally stimulates saliva and tear production
- Blurred vision, from the pupil muscles not contracting properly
- Constipation, from slowed movement in the digestive tract
- Difficulty urinating, particularly in men with enlarged prostates
- Dizziness and thickened mucus in the airways
These effects tend to be more pronounced at higher doses and in people who are sensitive to anticholinergic medications. For most healthy adults taking occasional standard doses, the side effects are mild and temporary.
Risks for Older Adults
Diphenhydramine is on the American Geriatric Society’s Beers Criteria, a widely used list of medications that older adults should generally avoid. The reason is that its brain-penetrating, acetylcholine-blocking properties pose specific dangers as people age. In older adults, diphenhydramine is associated with increased sedation, cognitive impairment, confusion, and a higher risk of falls. It should be particularly avoided in anyone experiencing delirium. If an older adult does need to take it for a specific reason, closer monitoring is recommended to prevent falls and other adverse outcomes.
Interactions With Alcohol and Sedatives
Mixing diphenhydramine with alcohol is a common and potentially dangerous combination. Alcohol enhances diphenhydramine’s sedating effects on the brain through a separate mechanism, meaning the two together produce more drowsiness, slower reaction times, and worse motor control than either one alone. This interaction is even more pronounced in older people.
The same caution applies to other sedating substances. Combining diphenhydramine with prescription sleep medications, anti-anxiety drugs, opioid painkillers, or even herbal sleep supplements can amplify sedation to dangerous levels. The effects aren’t just additive; in some cases they’re synergistic, meaning the combined impairment is greater than you’d expect from simply stacking two sedatives together.
Who Should Be Cautious
Because diphenhydramine blocks acetylcholine throughout the body, certain conditions make it riskier to use. People with narrow-angle glaucoma should avoid it, as it can increase pressure inside the eye. Those with an enlarged prostate may find that it worsens urinary retention. People with asthma or chronic lung disease should use it carefully, since it can thicken airway secretions. And anyone with a heart rhythm disorder should be aware that diphenhydramine affects the electrical signaling in heart tissue at higher doses, acting as a sodium channel blocker.
For children, over-the-counter diphenhydramine products carry specific age restrictions on their labels. Young children are particularly sensitive to its effects, and parents should follow age-based dosing guidelines on the packaging carefully rather than estimating doses.