Diphenhydramine
First-generation sedating antihistamine and the most common OTC sleep aid. Effective for occasional short-term use but tolerance to the hypnotic effect develops within days, and its strong anticholinergic action makes it a poor choice for regular or elderly use.
01 Overview
Diphenhydramine (brands Benadryl, and the sleep products Nytol, ZzzQuil, and the 'PM' component of many analgesics) is a first-generation H1 antihistamine widely sold OTC as a sleep aid. It crosses the blood-brain barrier readily, producing sedation via central H1 blockade.
It reliably induces drowsiness for occasional use, but tolerance to the sedative effect develops rapidly, often within three to four nights, limiting its value for chronic insomnia. Its pronounced anticholinergic activity produces next-day grogginess and cognitive dulling, and cumulative anticholinergic exposure is associated with increased dementia risk in older adults, so it is generally discouraged for the elderly.
02 Mechanism
Crosses the blood-brain barrier to antagonise central histamine H1 receptors, suppressing histaminergic arousal; also blocks muscarinic acetylcholine receptors, producing anticholinergic effects.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Sleep aid | 25–50 mg | Oral | 25-50 mg ~30 min before bed; for occasional use only. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Sedation / reduced sleep onset latencyReliably induces drowsiness and shortens time to fall asleep for occasional use. | Clinical | ||
| Antihistamine / anti-allergy effectPeripheral H1 blockade relieves allergic symptoms and itching, its original indication. | Clinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Rapid tolerance to hypnotic effectSedative efficacy diminishes markedly within 3-4 nights of continuous use. | Moderate | Nearly universal with nightly use | Clinical | |
| Anticholinergic burden / next-day grogginessDry mouth, constipation, blurred vision, urinary retention, and next-day cognitive dulling. | Moderate | Common | Clinical | |
| Delirium / confusion (overdose or elderly)Anticholinergic delirium, hallucinations, and agitation at high doses or in vulnerable individuals. | Severe | Uncommon | Clinical |