25 to 50 mg every 4 to 6 hours. Maximum 300 mg in 24 hours for adults and children 12 and over.
| Adults and 12+ | |
|---|---|
| Single dose | 25–50 mg |
| Interval | 4–6 hours |
| Doses per day | No more than 6 |
| Daily maximum | 300 mg |
| Standard tablet | 25 mg |
What it is used for, and what it is not
Diphenhydramine is a first-generation antihistamine. It treats allergic symptoms — hay fever, hives, itching, insect bites, mild allergic reactions — and it is used for motion sickness and, because of the sedation, as a sleep aid.
It does not treat anaphylaxis. Facial swelling, hives spreading rapidly, difficulty breathing or collapse is an emergency requiring adrenaline, and an oral antihistamine acts far too slowly to matter.
It does not treat a cold, though it appears in many cold products for its drying and sedating effects.
The sedation lasts longer than the effect
Diphenhydramine crosses into the brain readily, which is why it sedates and why the newer antihistamines — cetirizine, loratadine, fexofenadine — largely replaced it for daytime allergy use.
Its half-life is long enough that impairment commonly persists into the following morning, and driving studies have repeatedly found next-day impairment after an evening dose. That matters most for anyone taking it as a nightly sleep aid.
Tolerance to the sedative effect also develops within days, so it becomes less effective as a sleep aid while the next-day impairment continues.
Why it is no longer recommended in older adults
Several major geriatric guidelines, including the Beers Criteria, list diphenhydramine among the drugs to avoid in adults over 65.
The reasons are its strong anticholinergic activity: confusion, dry mouth, constipation, urinary retention, blurred vision, and an increased risk of falls. Some observational work has associated long-term cumulative anticholinergic exposure with cognitive decline.
For an older adult with allergy symptoms, a second-generation antihistamine is the usual alternative and does not carry the same profile.
Check the box, not the brand
Diphenhydramine appears under many names and inside many combination products — night-time cold and flu remedies, "PM" pain relievers, sleep aids, and allergy combinations.
Someone taking a PM painkiller and a night-time cold remedy can double the dose without exceeding the label of either. Read the active ingredient panel on everything, which is the same discipline that prevents most acetaminophen overdoses — and the two frequently appear in the same tablet.
When to ask before taking it
Glaucoma, enlarged prostate or urinary retention, asthma or chronic lung disease, an overactive thyroid, heart disease or high blood pressure, epilepsy, liver or kidney disease, pregnancy or breastfeeding, and anyone taking sedatives, opioids, or MAO inhibitors.
Alcohol compounds the sedation substantially and the combination should be avoided.
The newer antihistamines, and when the old one still wins
Cetirizine, loratadine and fexofenadine cross into the brain far less, last around 24 hours on a single daily dose, and cause a fraction of the sedation. For ongoing allergy symptoms they are the better choice on almost every axis.
Diphenhydramine retains two practical advantages. It acts faster — 15 to 30 minutes against an hour or more — which matters for an acute reaction while waiting for care. And it is available in an injectable form used in clinical settings.
For a nightly antihistamine through hay fever season, the newer drugs. For a single dose after a wasp sting, the older one is reasonable. The choice is about the situation rather than about which is better in general.
Overdose
Excessive sedation or, paradoxically, agitation; a rapid heart rate; dilated pupils; confusion or hallucinations; seizures. Diphenhydramine overdose is genuinely dangerous rather than merely unpleasant, and it is a same-day emergency call.
The paradoxical reaction — stimulation rather than sedation — is well recognised and more common in children and older adults.
Sources: Manufacturer package labelling; American Geriatrics Society Beers Criteria; NHS antihistamine guidance.
General reference reproducing published label dosing. Not medical advice. Your pharmacist and doctor know your other medications and your history.