First-generation H1-antihistamine (phenothiazine)

Promethazine hydrochloride

Brand names: Phenergan, Sominex

Promethazine is a sedating (first-generation) antihistamine used for allergic conditions, nausea and vomiting (including in pregnancy and motion sickness), and short-term for insomnia.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

DOSAGE AND ADMINISTRATION Promethazine HCl Suppositories are contraindicated for children under 2 years of age (see WARNINGS - Black Box Warning and Use in Pediatric Patients ). Promethazine HCl Suppositories are for rectal administration only. Allergy - The average dose is 25 mg taken before retiring; however, 12.5 mg may be taken before meals and on retiring, if necessary. Single 25 mg doses at bedtime or 6.25 to 12.5 mg taken three times daily will usually suffice. After initiation of treatment in children or adults, dosage should be adjusted to the smallest amount adequate to relieve symptoms. The administration of promethazine hydrochloride in 25 mg doses will control minor transfusion …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2023-06-30. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Clinical monograph

How it works

It is an H1-antihistamine with marked sedative and antimuscarinic activity, and antiemetic effects.

Prescribing in practice

Sedation is prominent — useful for its sedative/antiemetic effects but a drawback for daytime use; antimuscarinic effects occur.
Avoid in children under 2 years (risk of respiratory depression) and use caution in the elderly.
Additive sedation with alcohol and other CNS depressants.

Monitoring

Review the benefit and sedation; no routine monitoring.

Counselling the patient

It commonly causes drowsiness — do not drive while affected and avoid alcohol.
It is for short-term use when taken for sleep.

Evidence & guidelines

A sedating antihistamine used for allergy, nausea (including in pregnancy) and short-term insomnia.

Reference: MHRA; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways