What is the recommended dosage of Phenergan (promethazine) syrup for a 4-year-old child weighing 18 kg?

Phenergan (Promethazine) Dosing for a 4-Year-Old Child Weighing 18 kg

For nausea and vomiting in this 4-year-old child weighing 18 kg, the recommended dose of Phenergan (promethazine) syrup is 9 mg (0.5 mg per pound of body weight, which equals approximately 0.5 mg/kg), given every 4-6 hours as needed. 1

Weight-Based Calculation

• The child weighs 18 kg (approximately 40 pounds) 1
• Using the FDA-approved dosing of 0.5 mg per pound of body weight: 40 pounds × 0.5 mg/pound = 20 mg per dose 1
• Alternatively, using metric conversion (approximately 0.5 mg/kg): 18 kg × 0.5 mg/kg = 9 mg per dose 1
• The dose should be adjusted to the age and weight of the patient and the severity of the condition being treated 1

Critical Safety Considerations

Age Restriction

• Promethazine is absolutely contraindicated in children under 2 years of age due to risk of fatal respiratory depression 1
• This 4-year-old patient is above the minimum age threshold and can safely receive promethazine 1

Dosing Range for Different Indications

• For nausea and vomiting: 0.5 mg per pound of body weight (approximately 9-20 mg for this child), repeated every 4-6 hours as necessary 1
• For allergies: 6.25 to 12.5 mg three times daily, adjusted to the smallest amount adequate to relieve symptoms 1
• For motion sickness: 12.5 to 25 mg twice daily 1
• For sedation: 12.5 to 25 mg at bedtime 1

Important Clinical Caveats

Adverse Effects to Monitor

• Promethazine can cause significant sedation, agitation, hallucinations, seizures, and dystonic reactions in children 2
• If acute dystonic reactions occur, diphenhydramine (1-2 mg/kg or 25-50 mg/dose parenterally) can be administered 3
• The drug has been associated with serious adverse events, particularly when used inappropriately in young children 2, 4

Appropriate Use Guidelines

• Antiemetics should not be used in vomiting of unknown etiology in children and adolescents 1
• Establish the cause of vomiting before initiating treatment 1
• Start with the lowest effective dose and adjust based on response 1
• Doses may be repeated every 4-6 hours as necessary, but should not exceed recommended frequency 1

Alternative Routes

• If oral medication cannot be tolerated, promethazine can be given by rectal suppository 1
• Parenteral administration is also available when oral/rectal routes are not feasible 1