What is the recommended dosage of promethazine (Phenergan) for adults and children?

Promethazine Dosage Recommendations

For adults, promethazine is typically dosed at 12.5-25 mg orally, intravenously, intramuscularly, or rectally every 4-6 hours as needed, with lower doses (6.25-12.5 mg IV) being equally effective for antiemetic purposes while causing less sedation; for children over 2 years of age, the dose is 0.5 mg/kg (12.5-25 mg) every 4-6 hours, with promethazine contraindicated in children under 2 years due to risk of fatal respiratory depression. 1

Adult Dosing by Indication

Nausea and Vomiting

• Standard dose: 25 mg orally, with repeat doses of 12.5-25 mg every 4-6 hours as necessary 1
• Lower-dose alternative: 6.25-12.5 mg IV is equally effective for antiemetic purposes and causes significantly less sedation 2, 3
• Onset of action: Within 5 minutes IV, within 20 minutes orally 2
• Duration: 4-6 hours after a single dose, though effects may persist up to 12 hours 2

Allergic Conditions

• Standard regimen: 25 mg at bedtime, or 12.5 mg before meals and at bedtime if necessary 1
• Alternative: 6.25-12.5 mg three times daily after initiation 1
• Minor transfusion reactions: 25 mg single dose 1

Motion Sickness

• Prophylaxis: 25 mg twice daily, with initial dose taken 30-60 minutes before travel and repeated 8-12 hours later 1
• Subsequent days: 25 mg on arising and before evening meal 1

Sedation

• Nighttime/presurgical: 25-50 mg at bedtime 1
• Preoperative: 50 mg the night before surgery, combined with appropriately reduced narcotic/barbiturate doses 1
• Postoperative: 25-50 mg with reduced analgesic doses 1

Adjunct to Haloperidol for Agitation

• Combination therapy: 12.5-25 mg IV infused slowly, with total dose of 25-50 mg as adjuvant to haloperidol 2
• Mechanism: Provides protection against extrapyramidal side effects while adding sedative and antiemetic properties 2

Pediatric Dosing (Children ≥2 Years Only)

Critical Safety Warning

• Absolute contraindication: Promethazine is contraindicated in children under 2 years of age due to risk of fatal respiratory depression 1, 4
• Black box warning: Do not use in children under 2 years 1

Weight-Based Dosing

• General formula: 0.5 mg/kg per dose, adjusted to age, weight, and severity of condition 1
• Practical range: 12.5-25 mg every 4-6 hours for most indications 1

Specific Indications

• Nausea/vomiting: 12.5-25 mg every 4-6 hours as needed 1
• Motion sickness: 12.5-25 mg twice daily 1
• Allergies: 12.5-25 mg twice daily 1
• Sedation: 12.5-25 mg at bedtime 1
• Preoperative: 0.5 mg/kg combined with reduced narcotic/barbiturate doses 1

Route-Specific Considerations

Intravenous Administration

• Infusion rate: Must be administered slowly at ≤25 mg/min to minimize hypotension risk 2
• Tissue damage risks: Thrombophlebitis, tissue necrosis, and gangrene can occur with IV administration 2
• Preferred for acute settings: When oral medication cannot be tolerated 1

Intramuscular Administration

• Preferred site in adults: Deltoid muscle 5
• Preferred site in infants/young children: Anterolateral thigh (vastus lateralis muscle) due to low risk of vascular or nerve damage 5

Oral Administration

• Equivalent to parenteral: No advantage of IV/IM over oral route if GI absorption is intact 6
• Bioavailability consideration: Enteric promethazine has 25% bioavailability, yet current dosing is identical across routes 3

Critical Safety Considerations

Respiratory Depression

• High-risk populations: Children, elderly, patients receiving concurrent opioids 2, 7
• Cumulative dosing risk: Risk increases with repeated dosing 2
• Monitoring: Respiratory depression can occur within 30 minutes even at recommended doses 7
• Case reports: Respiratory arrest has occurred in children receiving 0.07 ml/kg of meperidine-promethazine-chlorpromazine combination 7

Sedation

• Dose-dependent: Significant sedation occurs, particularly problematic with repeated dosing or opioid combination 2
• Lower-dose benefit: 6.25 mg IV causes significantly less sedation than 12.5 mg while maintaining equal efficacy 8

Extrapyramidal Effects

• Risk: Neuroleptic malignant syndrome and other extrapyramidal effects can occur 2
• Implication: Makes promethazine inappropriate for chronic use 2

Special Populations

• Elderly: Starting dose of 6.25 mg IV is as effective as higher doses with fewer adverse drug reactions 8
• Combination with opioids: Requires dose reduction of standard sedation agents 2

Practical Dosing Algorithm

1. Verify age: Confirm patient is ≥2 years old (absolute contraindication if younger) 1
2. Select route: Oral preferred if tolerated; IV for acute settings with slow infusion 1, 2
3. Choose dose:
   • Adults: Start with 12.5 mg IV or 25 mg PO for most indications 2, 1
   • Elderly: Start with 6.25 mg IV 8
   • Children ≥2 years: 0.5 mg/kg (12.5-25 mg range) 1
4. Adjust to minimum effective dose: Titrate to smallest amount adequate to relieve symptoms 1
5. Repeat dosing: Every 4-6 hours as necessary, monitoring for respiratory depression 1, 2