diphenhydramine

General information

• Primary label indications include: Allergy symptoms (OTC labeling varies).
• Class: Adjunctive therapy
• Common US brands: BENADRYL
• Typical dose range: Allergy symptom dosing varies by product; one common pattern is 25–50 mg every 4–6 hours with a daily maximum (label varies).
• Therapeutic drug monitoring not routinely recommended.
• Last reviewed: 2026-02-12

Dosing & forms

• Forms/strengths: Common OTC tablets/capsules are 25 mg (product-dependent).; Included in many “PM” combination products; check labels to avoid duplication.
• Frequency: Allergy symptom dosing varies by product; one common pattern is 25–50 mg every 4–6 hours with a daily maximum (label varies).
• Typical range: Allergy symptom dosing varies by product; one common pattern is 25–50 mg every 4–6 hours with a daily maximum (label varies).

Mechanism (brief)

First-generation antihistamine (H1 antagonist) with sedating and anticholinergic effects; commonly used OTC for allergies and as a sleep aid.

Metabolism & Half‑life

• Metabolism: CYP2D6
• Half‑life: Single-dose mean 9 h

Therapeutic Drug Monitoring (TDM)

Recommended: No

Monitoring highlights

• Sedation and next-day impairment (falls risk, driving safety).
• Anticholinergic effects (constipation, urinary retention, blurred vision).
• Confusion or delirium risk in older adults and polypharmacy (Beers).
• Duplicate diphenhydramine exposure from “PM” combination products.

Sources

• Benadryl (diphenhydramine) tablets (OTC) label — DailyMed (2026)
• Diphenhydramine (StatPearls) — StatPearls Publishing (NCBI Bookshelf) (2025)
• Clinical Practice Guideline for the Pharmacologic Treatment of Chronic Insomnia in Adults: An American Academy of Sleep Medicine Clinical Practice Guideline — Journal of Clinical Sleep Medicine (2017)
• 2023 American Geriatrics Society Updated Beers Criteria for Potentially Inappropriate Medication Use in Older Adults — Journal of the American Geriatrics Society (2023)