Phenergan (Promethazine) Dosing
The recommended dosage of Phenergan (promethazine) varies by indication: for allergies, 12.5-25 mg orally at bedtime or 6.25-12.5 mg three times daily; for nausea/vomiting, 12.5-25 mg every 4-6 hours as needed (0.5 mg/lb in children); for motion sickness, 25 mg twice daily starting 30-60 minutes before travel; and for sedation, 25-50 mg at bedtime in adults or 12.5-25 mg in children. 1
Dosing by Indication
Allergic Conditions
- Standard dose: 25 mg orally at bedtime, or 12.5 mg before meals and at bedtime if necessary 1
- Maintenance: 6.25-12.5 mg three times daily typically suffices after initial treatment 1
- Transfusion reactions: 25 mg controls minor allergic transfusion reactions 1
Nausea and Vomiting
- Adults: 25 mg as the effective dose, repeated every 4-6 hours as needed 1
- Children: 0.5 mg per pound of body weight, adjusted for age and severity 1
- Prophylaxis (surgical/postoperative): 25 mg repeated at 4-6 hour intervals 1
- Alternative low-dose approach: Research demonstrates that 6.25 mg IV is as effective as ondansetron 4 mg for nausea relief (74% vs 59% at 1 hour) with comparable sedation profiles 2
Motion Sickness
- Adults: 25 mg twice daily 1
- Timing: Initial dose 30-60 minutes before travel, repeat 8-12 hours later 1
- Subsequent days: 25 mg on arising and before evening meal 1
- Children: 12.5-25 mg twice daily 1
Sedation
- Adults: 25-50 mg at bedtime for nighttime, presurgical, or obstetrical sedation 1
- Children: 12.5-25 mg at bedtime 1
Preoperative Use
- Night before surgery: 12.5-25 mg for children, 50 mg for adults 1
- Preoperative medication: Children require 0.5 mg/lb combined with reduced narcotic/barbiturate doses plus atropine-like drug 1
- Adults: 50 mg with reduced narcotic/barbiturate and belladonna alkaloid 1
- Postoperative: 12.5-25 mg in children, 25-50 mg in adults 1
Route-Specific Considerations
Oral Administration
- Preferred route for chronic management 1
- Bioavailability is approximately 25% compared to parenteral routes 2
Intramuscular Administration
- Preferred parenteral route to avoid vascular complications 3
- Deep IM injection recommended 3
- Absorption is slow; 6 of 10 subjects reported drowsiness at 6 hours post-injection 4
Intravenous Administration
- High risk: Promethazine is highly caustic to vessel intima and surrounding tissues 3
- Complications: May cause thrombophlebitis, inadvertent intra-arterial injection, perivascular extravasation, and tissue necrosis 3
- Safety measures if IV route necessary: Use large, patent veins; dilute the drug; administer slowly; use lower doses 3
- Low-dose IV alternative: 6.25-12.5 mg IV provides equivalent antiemetic efficacy to standard 25 mg dose with reduced sedation risk 2
Critical Safety Warnings
Black Box Warning - Pediatric Use
- Contraindicated in children under 2 years of age 1
- Respiratory depression risk, particularly when combined with other sedatives 1
Administration Precautions
- Avoid rapid IV administration due to pain and seizure risk 5
- Monitor vital signs and oxygen saturation during and after administration 5
- Be prepared for respiratory support, especially with concomitant sedative use 5
Drug Interactions
- Contraindicated: Monoamine oxidase inhibitors (MAOIs) 1
- Avoid: Other sympathomimetic amines, anticholinergic medications 1
- Caution: Concurrent use with narcotic analgesics significantly increases sedation 4
Contraindications
- Pregnancy (especially first trimester) 5
- Angle-closure glaucoma 5
- Prostatic hypertrophy and urinary retention 5
- Severe hepatic disease (sedation may precipitate hepatic coma) 5
- Lactation 5
Common Pitfalls to Avoid
Dosing Errors
- Do not use identical dosing for all routes: Despite similar dosing recommendations in references, IV bioavailability differs significantly from oral 2
- Adjust dose to minimum effective amount after initiating treatment 1
Administration Errors
- Never use as monotherapy for anaphylaxis: Epinephrine must be given first; promethazine is adjunctive second-line therapy only 5
- Avoid IV route when possible: IM injection is safer and avoids thrombophlebitis risk 3
Monitoring Failures
- Watch for paradoxical agitation, particularly in pediatric patients 5
- Anticipate sedation as the most common side effect 5
- Monitor for hypotension, especially with rapid administration 5