Promethazine DM Dosing Guidelines
For adults, promethazine DM should be dosed at 6.25-12.5 mg of promethazine with appropriate dextromethorphan (DM) every 4-6 hours as needed, with a maximum of 50 mg of promethazine in 24 hours. 1
Adult Dosing
- Standard dosing: 12.5-25 mg every 4-6 hours
- Starting dose: 6.25-12.5 mg, especially in elderly or those sensitive to sedative effects
- Maximum daily dose: 25-50 mg in 24 hours
- Timing: Can be taken before meals and at bedtime as needed
Special Populations
Elderly Patients
- Start with lowest dose: 6.25 mg due to increased sensitivity to anticholinergic effects 2
- Use with caution: The American Geriatrics Society cautions against using first-generation antihistamines like promethazine in older adults due to significant risks of anticholinergic effects, sedation, confusion, and falls 2
- Consider alternatives: Non-sedating antiemetics like ondansetron 4 mg may be safer for elderly patients 3
Pediatric Patients
- Contraindicated: Absolutely contraindicated in children under 2 years of age due to risk of fatal respiratory depression 1
- Children over 2 years: 0.5 mg/kg/dose or 12.5-25 mg twice daily 1
- Monitor closely: Respiratory depression has been reported even at recommended doses 4
Administration Considerations
- Take with food to reduce gastrointestinal irritation 2
- For nausea and vomiting: 12.5-25 mg doses may be repeated at 4-6 hour intervals as needed 1
- For sedation: 12.5-25 mg at bedtime 1
Precautions and Monitoring
- Respiratory depression: Monitor respiratory rate, especially when combined with other CNS depressants 4
- Anticholinergic effects: Watch for dry mouth, urinary retention, blurred vision, and cognitive impairment 2
- Sedation: Significant sedative effects can occur, especially when combined with opioids or other CNS depressants 3
- Cardiovascular effects: Minimal effects on autonomic cardiovascular mechanisms have been reported 5
Clinical Pearls
- Low-dose promethazine (6.25 mg IV) has been shown to be as effective as ondansetron 4 mg for nausea and vomiting with similar sedation profiles 3
- Promethazine exhibits linear dose proportionality, meaning that doubling the dose doubles the blood concentration 6
- When using for antiemetic purposes, lower doses (6.25-12.5 mg) may provide adequate relief while minimizing sedative effects 3
Common Pitfalls to Avoid
- Exceeding maximum daily dose of 50 mg
- Using standard adult dosing in elderly patients, which significantly increases adverse effect risk
- Combining with other CNS depressants without dose adjustment
- Using in patients with significant respiratory conditions where respiratory depression could be dangerous
- Failing to monitor for anticholinergic side effects in susceptible populations