Promethazine (Phenergan) Dosing Guidelines
The recommended dosing for Phenergan (promethazine) varies by indication, with adults typically receiving 12.5-25 mg orally every 4-6 hours as needed, while lower doses of 6.25 mg IV are equally effective with fewer side effects for nausea and vomiting. 1, 2
Adult Dosing by Indication
Allergy
- Standard dose: 25 mg at bedtime
- Alternative regimen: 12.5 mg three times daily with meals 1
- Adjust to smallest effective dose after symptom control
Motion Sickness
- Standard dose: 25 mg twice daily 1
- First dose: 30-60 minutes before travel
- Second dose: 8-12 hours later if needed
- On subsequent travel days: 25 mg upon waking and before evening meal
Nausea and Vomiting
- Standard dose: 12.5-25 mg every 4-6 hours as needed 1
- Low-dose option: 6.25 mg IV (recommended for elderly patients) 2, 3
- Important: Do not use for vomiting of unknown etiology in children/adolescents
Sedation
- Adults: 25-50 mg at bedtime 1
- Pre/post-operative:
- Night before surgery: 50 mg
- Preoperative: 50 mg (with reduced narcotic/barbiturate dose)
- Postoperative: 25-50 mg
Pediatric Dosing
- Contraindicated in children under 2 years (Black Box Warning) 1
- Motion sickness: 12.5-25 mg twice daily
- Nausea/vomiting: 0.5 mg per pound of body weight
- Sedation: 12.5-25 mg at bedtime
- Pre/post-operative: 0.5 mg per pound of body weight
Route of Administration Considerations
Oral Administration
- Preferred route when tolerated
- Full effect may take 20 minutes
Intravenous Administration
- Key finding: Lower doses (6.25-12.5 mg) are as effective as standard doses with fewer side effects 2, 3
- Administer slowly over 10-15 minutes to minimize adverse effects
- Dilute in compatible solution to reduce vein irritation
Special Populations
Elderly Patients
- Start with 6.25 mg IV - equally effective as higher doses with significantly fewer adverse effects 3
- Monitor closely for sedation, confusion, and anticholinergic effects
- Increase dose only if necessary for symptom control
Patients with Liver Impairment
- Start with lower doses (12.5 mg)
- Monitor for excessive sedation
Important Considerations and Precautions
Potential Adverse Effects
- Sedation (most common)
- Anticholinergic effects (dry mouth, urinary retention, blurred vision)
- Extrapyramidal symptoms (rare)
- Respiratory depression (especially when combined with opioids)
Contraindications
- Children under 2 years
- Patients with CNS depression
- Hypersensitivity to phenothiazines
- Monoamine oxidase inhibitor use (within 14 days)
Drug Interactions
- Enhanced sedation with CNS depressants (opioids, benzodiazepines)
- Increased anticholinergic effects with other anticholinergic medications
- Avoid alcohol
Clinical Pearls
- For nausea/vomiting, low-dose IV promethazine (6.25 mg) is as effective as ondansetron 4 mg with similar sedation profiles 2
- When using for sedation with opioids, lower doses (12.5 mg) provide optimal sedation without excessive respiratory depression 4
- For elderly patients, the 6.25 mg IV dose has significantly fewer adverse drug reactions than higher doses while maintaining efficacy 3