Promethazine: clinical use

Prescribing considerations

• Confirm the product, indication and age restrictions, which differ between formulations.
• Assess sedative and anticholinergic burden, falls risk, respiratory disease and driving or machinery use.
• Review cardiovascular disease, bradycardia, electrolyte disturbance, QT prolongation and concurrent QT-prolonging medicines.
• Use particular caution in older people because of confusion, hypotension and anticholinergic effects.
• Parenteral use requires trained administration because incorrect placement or extravasation can cause severe tissue injury.

Contraindications and cautions

• Hypersensitivity to promethazine, other phenothiazines or formulation ingredients.
• Coma or central nervous system depression.
• Recent monoamine oxidase inhibitor use, as defined by the product information.
• Age contraindications are formulation-specific.
• Avoid or carefully reassess use with narrow-angle glaucoma, urinary outflow obstruction, myasthenia gravis, Parkinson's disease, significant hepatic or renal dysfunction, seizure disorders and relevant cardiac disease.

Monitoring

• Monitor sedation, cognition, blood pressure, anticholinergic effects and respiratory status according to risk.
• Investigate unexplained fever, sore throat or infection promptly with a full blood count.
• Consider ECG and electrolyte assessment when QT-risk factors are present.
• Monitor the injection site closely; stop intravenous administration immediately if pain suggests extravasation or intra-arterial placement.
• Promethazine may suppress skin-test responses and interfere with immunological urine pregnancy tests.

Clinical pharmacology

Promethazine is a potent, long-acting H1-antihistamine phenothiazine with central sedative, antiemetic and anticholinergic activity. Oral promethazine undergoes extensive first-pass hepatic metabolism and crosses the blood–brain barrier and placenta.

Formulation and product differences

• Sominex is an oral tablet licensed specifically as a night-time aid for temporary sleep disturbance and contains lactose.
• Phenergan tablets have broader allergy, antiemetic and sedative indications and contain lactose.
• Phenergan injection is for healthcare-professional administration and includes perioperative uses. It contains sulphites that may rarely cause severe hypersensitivity or bronchospasm.
• Injection technique is safety-critical because incorrect administration can cause necrosis and peripheral gangrene.

Tablet

Route: Oral

Strengths: 25 mg

Injection

Route: Parenteral

Strengths: 25 mg/mL