promethazine

General information

• Class: Adjunctive therapy
• Common US brands: PHENERGAN
• Typical dose range: Dosing is indication-dependent (allergy vs nausea vs motion sickness vs sedation). Labels emphasize using the smallest dose adequate to relieve symptoms (label).
• Therapeutic drug monitoring not routinely recommended.
• Last reviewed: 2026-02-22

Dosing & forms

• Forms/strengths: Oral tablets: 12.5 mg, 25 mg, 50 mg (label).
• Frequency: Sedation labeling includes bedtime dosing (e.g., 25–50 mg in adults, product-dependent). Next-day impairment can occur; avoid stacking with other sedatives when possible (label/class).
• Typical range: Dosing is indication-dependent (allergy vs nausea vs motion sickness vs sedation). Labels emphasize using the smallest dose adequate to relieve symptoms (label).

Mechanism (brief)

Sedating first-generation antihistamine (H1 antagonist) and phenothiazine derivative with anticholinergic/antiemetic properties; used for allergy symptoms, nausea, motion sickness, and sedation.

Metabolism & Half‑life

• Metabolism: Hepatic
• Half‑life: —

Therapeutic Drug Monitoring (TDM)

Recommended: No

Monitoring highlights

• Sedation and next-day impairment (driving safety, falls risk).
• Respiratory depression risk when combined with other depressants or in sleep apnea.
• Anticholinergic effects (constipation, urinary retention, blurred vision) and delirium risk.
• Abnormal movements or rigidity/fever suggesting EPS or rare NMS.

Sources

• Promethazine hydrochloride tablets prescribing information — DailyMed (2025)
• Promethazine (StatPearls) — StatPearls Publishing (NCBI Bookshelf) (2025)
• 2023 American Geriatrics Society Updated Beers Criteria for Potentially Inappropriate Medication Use in Older Adults — Journal of the American Geriatrics Society (2023)
• Pharmacokinetics of promethazine and its sulphoxide metabolite after intravenous and oral administration to man — British Journal of Clinical Pharmacology (1983)