Promethazine

What is Promethazine used for?

Promethazine is a first-generation (sedating) antihistamine that relieves allergic symptoms by blocking H1 histamine receptors, and its sedative CNS effect also makes it useful for short-term insomnia, motion sickness, and post-operative nausea/vomiting. In Hong Kong it is also commonly included in compound cough/cold syrups as a sedating ingredient. Because sedation is significant, avoid driving or operating machinery after use.

How to take Promethazine and adjust the dose

Typical adult oral dosing is around 10-25mg, 1-3 times daily, or as directed; for motion sickness prevention, 25mg is commonly used in adults. Dosing for compound cough syrups varies by formulation strength — always follow the specific product label or prescription rather than assuming a single dose applies to all promethazine products. Contraindicated in children under 2 due to risk of respiratory depression (some fatal cases reported); Hong Kong's Drug Office has issued safety alerts on this. (Confidence: moderate — dosing varies significantly by formulation; defer to specific product labeling)

Promethazine side effects

• Common: drowsiness, dry mouth, dizziness
• More serious: involuntary muscle movements (especially facial), easy bruising/bleeding, fever/sore throat (rare blood dyscrasia signs)
• Rare and serious: neuroleptic malignant-type syndrome (muscle rigidity, high fever, confusion, irregular heartbeat)

Who should not take Promethazine?

• Contraindicated in children under 2 years (respiratory depression risk)
• Contraindicated in comatose states
• Contraindicated in known hypersensitivity to promethazine or other phenothiazines
• Use with caution/avoid in asthma or lower respiratory tract disease
• Elderly patients at higher risk of pronounced drowsiness/confusion
• Consult a doctor if pregnant or breastfeeding
• Avoid combining with alcohol or other CNS depressants