First-generation H1-antihistamine (phenothiazine) Pregnancy: eMC section 4.6: use is not recommended during pregnancy or in women of childbearing potential not using contraception unless the potential benefits outweigh the potential risks. High doses given in late pregnancy have caused prolonged neurological disturbances in the infant. Excreted in breast milk with risks of neonatal irritability and excitement — not recommended for use in breast-feeding.

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.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: As an antihistamine in allergy (adults and children over 10 years, including the elderly): 25 mg as a single dose, increasing to a maximum of 25 mg twice a day as required

Route: Oral

Frequency: Single dose, best taken at night; increased to twice daily as required

Max: 25 mg twice a day (allergy indication)

Source SPC is Phenergan 25 mg tablets (oral) — injectable promethazine posology is not covered by this source. Other indications per eMC section 4.2, for adults and children over 10 years including the elderly: as an antiemetic (motion sickness) — 25 mg to be taken the night before the journey, repeated after 6 to 8 hours as required; for short-term treatment of insomnia — 25 mg or 50 mg as a single night-time dose. Phenergan should not be used for longer than 7 days without seeking medical advice. PAEDIATRIC (stated as fixed doses, not weight-based, so not structured below): not for use in children under 6 years of age; children 6 to 10 years — as an antihistamine in allergy 25 mg as a single dose with a maximum daily dose of 25 mg; as an antiemetic in this age group the Elixir is recommended rather than tablets; as a sedative for short-term use 25 mg as a single night-time dose. Verify all paediatric use against a children's formulary. Section 4.5 was not captured in the fetched extract — the interaction list below is drawn from sections 4.3 and 4.4.

Dose adjustments

Renal

No numeric adjustment is stated. eMC section 4.4: Phenergan should be avoided in patients with liver or renal dysfunction.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm 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.

Contraindications

Known hypersensitivity to promethazine, to other phenothiazines, or to any other ingredient of the formulation
Patients in coma or suffering from CNS depression of any cause
Children less than 6 years of age
Should be avoided in patients taking monoamine oxidase inhibitors up to 14 days previously

Side effects

Sedation or somnolence (very common)
Anticholinergic effects — dry mouth, constipation, urinary retention, accommodation disorder, paralytic ileus; the elderly are particularly susceptible, including to confusion
Extrapyramidal effects including dystonia and oculogyric crisis, restless legs, muscle spasms and tic-like movements — commoner in children and young adults, usually within the first 4 days or after a dose increase
Cardiac — palpitations and arrhythmias including QT prolongation and torsade de pointes; hypotension
Respiratory depression; nasal congestion
Blood dyscrasias including haemolytic anaemia, agranulocytosis, leukopenia, eosinophilia and thrombocytopenia; cholestatic jaundice; rash and photosensitivity reaction

Interactions

Other sedative medicinal products — combined use is not recommended because of the risk of sedation
Alcohol and alcohol-containing medicines should be avoided while taking this medicine
Other CNS depressants (opiates and other analgesics, barbiturates and other sedatives, general anaesthetics) — phenothiazines may be additive with, or may potentiate, their action
Ototoxic drugs (e.g. salicylates) — promethazine may mask the warning signs of ototoxicity
Drugs causing bradycardia, hypokalaemia or acquired QT prolongation — phenothiazine derivatives may potentiate QT prolongation, increasing the risk of torsade de pointes

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. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways