What is the recommended dosing for promethazine (Phenergan)?

How to Write an Order for Promethazine

For adult patients, order promethazine 12.5-25 mg IV infused slowly over 10 minutes, with a maximum total dose of 25-100 mg when used as an adjuvant to narcotics and benzodiazepines; never administer to children under 2 years of age due to risk of fatal respiratory depression. 1, 2, 3

Route-Specific Dosing

Intravenous Administration

• Standard IV dose: 12.5-25 mg infused slowly over 10 minutes to minimize hypotension risk 1, 2
• Total dose range: 25-100 mg when used as adjuvant therapy with narcotics and benzodiazepines 1
• Onset of action occurs within 5 minutes, with duration of 4-6 hours and plasma half-life of 9-16 hours 1, 2
• Critical safety consideration: Slow infusion is mandatory—rapid IV administration significantly increases hypotension risk 1, 2
• Research supports that lower doses (6.25 mg IV) may be equally effective as standard dosing while potentially reducing sedation, though this is not FDA-approved 4

Oral Administration

• Standard oral dose: 12.5-25 mg every 4-6 hours for nausea/vomiting 3, 5
• For allergies: 25 mg at bedtime, or 12.5 mg before meals and at bedtime 3, 5
• For motion sickness: 25 mg twice daily, with initial dose 30-60 minutes before travel, repeated 8-12 hours later 3, 5
• For sedation: Adults require 25-50 mg at bedtime 3, 5

Rectal Administration

• Dosing identical to oral route: 12.5-25 mg every 4-6 hours 3, 5
• Use when oral medication cannot be tolerated 3, 5
• Rectal suppositories produce lower peak concentrations (Cmax) and delayed time to peak (6.7-8.6 hours vs 4.4 hours for oral) but comparable overall bioavailability (70-97%) 6

Indication-Specific Dosing

Nausea and Vomiting

• First-line dose: 12.5-25 mg PO/IV/PR every 4-6 hours 1, 2, 3
• Use when ondansetron or other 5-HT3 antagonists fail 2
• For prophylaxis during surgery: 25 mg repeated at 4-6 hour intervals 3, 5
• Pediatric dosing (>2 years): 0.5 mg per pound body weight, adjusted for age and severity 3, 5

Sedation and Procedural Use

• Preoperative sedation: 50 mg the night before surgery 3, 5
• Procedural sedation: 12.5-25 mg as adjuvant to narcotics and benzodiazepines 1, 2
• Postoperative sedation: 25-50 mg in adults, 12.5-25 mg in children >2 years 3, 5

Critical Safety Warnings

Absolute Contraindications

• NEVER administer to children under 2 years of age—risk of fatal respiratory depression 2, 3, 5
• Do not use in vomiting of unknown etiology in children and adolescents 3, 5

Administration Precautions

• Extravasation risk: Inadvertent perivascular extravasation, intra-arterial injection, or intraneuronal infiltration can cause severe local tissue damage including burning, pain, thrombophlebitis, tissue necrosis, and gangrene 1
• Monitor for hypotension, particularly with rapid IV infusion 1, 2
• Be prepared for respiratory depression, especially when combined with opioids or benzodiazepines 2
• Watch for neuroleptic malignant syndrome and extrapyramidal effects 1, 2

Common Adverse Effects

• CNS depression and sedation occur frequently—may be advantageous when sedation is desired 2
• Dizziness, blurred vision, and dry mouth are common 2
• Extrapyramidal symptoms can develop similar to other dopamine antagonists 2

Sample Order Writing

For nausea/vomiting in adults: "Promethazine 12.5-25 mg IV over 10 minutes every 4-6 hours as needed for nausea/vomiting. Maximum 100 mg per 24 hours."

For preoperative sedation: "Promethazine 50 mg PO tonight at bedtime for preoperative sedation."

For procedural sedation: "Promethazine 12.5-25 mg IV over 10 minutes as adjuvant sedation for procedure."