What is the recommended weight-based dose of promethazine (Phenergan) for a pediatric patient over 2 years old?

Weight-Based Dosing of Promethazine in Pediatric Patients Over 2 Years

For pediatric patients over 2 years of age, promethazine should be dosed at 0.5 mg per pound of body weight (approximately 1.1 mg/kg) for nausea and vomiting, with individual doses of 12.5-25 mg repeated every 4-6 hours as necessary, adjusted to the age, weight, and severity of the condition. 1

Critical Safety Considerations

Promethazine is absolutely contraindicated in children under 2 years of age due to fatal respiratory depression risk, and this contraindication applies to all formulations and indications. 2

• Avoid promethazine in children with respiratory compromise or those receiving other respiratory depressants, as the risk of respiratory depression is particularly concerning when combined with other sedating medications 2
• Significant sedation occurs with promethazine, particularly problematic with repeated dosing or when combined with opioids 3
• The risk of respiratory depression increases with cumulative dosing 3

Specific Dosing by Indication

Nausea and Vomiting

• The standard weight-based dose is 0.5 mg per pound of body weight (approximately 1.1 mg/kg), adjusted to age, weight, and severity of condition 1
• Individual doses of 12.5-25 mg may be repeated every 4-6 hours as necessary 1
• Lower doses of 6.25-12.5 mg are equally effective for antiemetic purposes and cause less sedation 3
• Onset of action occurs within 20 minutes when given orally 3

Allergy

• Single 25 mg doses at bedtime, or 6.25-12.5 mg taken three times daily 1
• For children, 12.5 mg doses before meals and at bedtime may suffice 1

Motion Sickness

• For children, 12.5-25 mg twice daily 1
• Initial dose should be taken one-half to one hour before anticipated travel and repeated 8-12 hours later if necessary 1

Sedation

• 12.5-25 mg at bedtime provides sedation in children 1
• For preoperative medication, children require 0.5 mg per pound of body weight in combination with appropriately reduced doses of narcotic or barbiturate 1

Route of Administration

Oral administration of promethazine is equivalent to parenteral administration if GI absorption is intact. 3

• When oral medication cannot be tolerated, the dose may be given parenterally or by rectal suppository 1
• Enteric promethazine has 25% bioavailability, yet current dosing is identical across routes 3

Duration and Monitoring

• Duration of action is 4-6 hours after a single dose, though effects may persist up to 12 hours 3
• Plasma half-life is 9-16 hours 3
• After initiation of treatment, dosage should be adjusted to the smallest amount adequate to relieve symptoms 1

Common Pitfalls to Avoid

• Never use promethazine chronically due to risk of extrapyramidal effects, including neuroleptic malignant syndrome 3
• Avoid repeated or prolonged courses due to tissue damage risks such as thrombophlebitis, tissue necrosis, and gangrene with IV administration 3
• Do not exceed recommended dosing intervals, as cumulative dosing increases respiratory depression risk 3
• When IV administration is necessary, infuse slowly (≤25 mg/min) to minimize hypotension risk 3