Promethazine

How it works

It antagonises histamine H1 receptors and also blocks muscarinic and central receptors, producing sedation, antiemesis and relief of histamine-mediated symptoms such as pruritus.

Prescribing in practice

• Its marked sedative effect is additive with opioids and other central depressants, so use cautiously where these are co-administered and monitor for excessive sedation and respiratory depression.
• Anticholinergic effects can cause urinary retention, dry mouth, confusion and worsening of narrow-angle glaucoma, with the elderly particularly susceptible to confusion and falls.
• Promethazine should be avoided in young children because of the risk of respiratory depression; in burns, itch from healing wounds is better managed with non-sedating measures where possible.

Monitoring

Monitor sedation level, anticholinergic effects and respiratory status, especially in older patients and those on concomitant opioids.

Counselling the patient

• This can help with itching, sickness or sleep but is quite sedating.
• Take care with drowsiness, particularly alongside strong painkillers.
• Report difficulty passing urine, marked confusion or excessive sleepiness.

Evidence & guidelines

Promethazine's sedative and anticholinergic profile and its avoidance in young children are detailed in current prescribing references and MHRA advice on sedating antihistamines.