Promethazine DM Dosage
For promethazine-dextromethorphan combination products, the standard adult dosage is 12.5-25 mg of promethazine taken twice daily, with the initial dose taken one-half to one hour before anticipated need and repeated 8-12 hours later if necessary; for children over 2 years of age, 12.5-25 mg twice daily may be administered, though this medication is contraindicated in children under 2 years of age. 1
Adult Dosing
- Standard dose: 25 mg taken twice daily for most indications including allergy and motion sickness 1
- Nausea/vomiting: 12.5-25 mg repeated as necessary at 4-6 hour intervals 1
- Sedation: 25-50 mg for nighttime sedation 1
- Maximum infusion rate: When given intravenously, promethazine must be infused slowly (≤25 mg/min) to minimize hypotension risk 2, 3
Pediatric Dosing (>2 Years Only)
- Motion sickness/allergy: 12.5-25 mg twice daily 1
- Nausea/vomiting: 0.5 mg per pound of body weight, adjusted to age and severity 1
- Sedation: 12.5-25 mg at bedtime 1
- Absolute contraindication: Children under 2 years of age due to risk of respiratory depression and sudden death 1, 4
Critical Safety Considerations
Promethazine carries significant risks that limit its appropriate use:
- Respiratory depression increases with cumulative dosing, particularly when combined with opioids or other CNS depressants 3, 4
- Tissue damage risks including thrombophlebitis, tissue necrosis, and gangrene are associated with IV administration 3
- Extrapyramidal effects ranging from restlessness to oculogyric crises and neuroleptic malignant syndrome can occur 2, 3
- Significant sedation is problematic with repeated dosing or opioid co-administration 3
Pharmacokinetic Profile
- Onset of action: 5 minutes IV, 20 minutes orally 3
- Duration: 4-6 hours after single dose, though effects may persist up to 12 hours 3
- Half-life: 9-16 hours 2, 3
- Peak concentration: Occurs at 4.4 hours for oral syrup, 6.7-8.6 hours for suppositories 5
Common Pitfalls to Avoid
- Do not use in children under 2 years - this is an absolute contraindication due to fatal respiratory depression risk 1, 4
- Avoid chronic or prolonged use - extrapyramidal effects and tissue damage risks make this inappropriate 3
- Exercise extreme caution with opioid combinations - the dextromethorphan component combined with promethazine's sedative effects creates significant respiratory depression risk, and this combination has been associated with drug-related fatalities 6
- Be aware of serotonin syndrome risk - dextromethorphan can interact with SSRIs to cause serotonin syndrome 7
- Adjust dosing to minimum effective amount - after initiation, dosage should be adjusted to the smallest amount adequate to relieve symptoms 1
Drug Interaction Warning
When promethazine-dextromethorphan is used in patients on SSRIs or other serotonergic medications, monitor closely for serotonin syndrome (altered mental status, myoclonus, autonomic instability) as dextromethorphan is a serotonin reuptake inhibitor 7