Diphenhydramine
- Class: First-generation antihistamine
- Used for: allergies and occasional sleeplessness
- Say it: dye-fen-HYE-druh-meen
- Forms: tablet, capsule, oral liquid, injection
DIPHENHYDRAMINE = DEEP sleep and a DRY mouth. It blocks histamine everywhere, including the brain.
Both halves matter. The deep is the sedation it is often taken for; the dry is the anticholinergic load that causes retention, blurred vision and, in an older patient, delirium.
Diphenhydramine crosses into the brain freely, which gives the sedation it is often taken for. It is also strongly anticholinergic — dry mouth, blurred vision, urinary retention and confusion all follow.
Side effects
Common: drowsiness, dry mouth, constipation
Serious: confusion in older adults, urinary retention
Typical dose
25–50 mg every 4–6 hours
Maximum 300 mg daily. The dose needed for sleep and the dose needed for allergy are much the same.
Check yourself
Why is diphenhydramine a poor choice in an older patient?
- Its strong anticholinergic effects cause confusion, retention and falls
- It does not block histamine effectively
- It interacts with every antibiotic
- It raises blood pressure dangerously
Show answer
A. Its strong anticholinergic effects cause confusion, retention and falls
The antihistamine effect is not the problem — the off-target muscarinic blockade is. Older brains tolerate it badly, and the resulting delirium is often investigated at length before anyone asks about the sleep aid.
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