What is the recommended dosage of intramuscular (IM) Phenergan (promethazine)?

IM Phenergan (Promethazine) Dosage

The recommended IM dose of promethazine for adults is 25 mg, which may be repeated within 2 hours if necessary for allergic conditions, or 12.5-25 mg every 4 hours for nausea/vomiting, with deep intramuscular injection being the preferred and safest parenteral route. 1

Adult Dosing by Indication

Allergic Conditions

• Standard dose: 25 mg IM 1
• May repeat within 2 hours if necessary 1
• Transition to oral route as soon as circumstances permit 1
• For allergic reactions to blood or plasma: 25 mg IM 1

Nausea and Vomiting

• Dose: 12.5-25 mg IM 1
• Do not repeat more frequently than every 4 hours 1
• When used postoperatively, reduce concomitant analgesic and barbiturate doses accordingly 1

Sedation (Hospitalized Patients)

• Nighttime sedation: 25-50 mg IM 1

Preoperative/Postoperative Use

• Dose: 25-50 mg IM 1
• Combine with appropriately reduced doses of analgesics and atropine-like drugs 1
• Reduce concomitant analgesic or hypnotic medication accordingly 1

Obstetric Use

• Early labor sedation: 50 mg IM 1
• Established labor: 25-75 mg IM (average 50 mg) with reduced narcotic dose 1
• May repeat once or twice at 4-hour intervals if necessary 1
• Maximum total dose: 100 mg in 24 hours during labor 1

Pediatric Dosing

Critical Age Restrictions

• Contraindicated in children under 2 years of age 1
• For children ≥2 years: use lowest effective dose and avoid concomitant respiratory depressants 1

Administration Technique and Safety

Route Selection Priority

• Deep intramuscular injection is the preferred parenteral route 1
• Subcutaneous injection is contraindicated due to tissue necrosis risk 1
• Under no circumstances give intra-arterially due to severe arteriospasm and gangrene risk 1

IV Administration (When Necessary)

• Maximum concentration: 25 mg/mL 1
• Maximum rate: 25 mg per minute 1
• Preferably inject through functioning IV infusion set tubing 1
• Stop immediately if patient complains of pain to evaluate for arterial injection or extravasation 1

Critical Safety Considerations

Tissue Injury Risk

• Promethazine can cause severe chemical irritation and tissue damage regardless of route 1
• Risks include perivascular extravasation, unintentional intra-arterial injection, and intraneuronal/perineuronal infiltration 1

Cardiovascular Effects

• Research demonstrates minimal effect on autonomic cardiovascular mechanisms at 50 mg IM dose 2
• No significant changes in resting heart rate, blood pressure, baroreflex function, or plasma catecholamines at 3 or 19.5 hours post-injection 2

Sedation and Performance Impact

• 25 mg IM promethazine without amphetamine produces performance decrements equivalent to blood alcohol level of 25-50 mg% (1-2 drinks) 3
• Drowsiness reported at 6 hours post-IM injection in 60% of subjects due to slow absorption 3
• Most subjects report drowsiness, with some experiencing nervous excitation, dry mouth, and fatigue 2

Drug Interactions

• Increased respiratory depression risk when combined with opioids or other CNS depressants 1
• Reduce analgesic and sedative doses when used concomitantly 1

Common Pitfalls to Avoid

• Never use IV route as first-line—IM is preferred and safer 1
• Never give subcutaneously—causes tissue necrosis 1
• Never exceed 4-hour dosing intervals for nausea/vomiting 1
• Do not use in vomiting of unknown etiology in children and adolescents 1
• Inspect solution before use; do not use if discolored or contains precipitate 1