Do Not Prescribe Promethazine DM to This Child
This child weighing 13.79 kg is contraindicated for promethazine DM because the FDA explicitly prohibits its use in children under 2 years of age, and based on typical weight-for-age, this weight suggests the child is likely under 2 years old. 1
Critical Age and Weight Assessment
A child weighing 13.79 kg typically falls into one of these categories:
- If under 2 years old: Absolutely contraindicated (FDA Black Box Warning)
- If 2 to under 6 years old: Dose would be 1.25-2.5 mL every 4-6 hours (maximum 10 mL/24 hours)
- If 6 to under 12 years old (unlikely at this weight): Dose would be 2.5-5 mL every 4-6 hours (maximum 20 mL/24 hours)
Dosing Algorithm
Step 1: Verify the child's age first
- If age < 2 years: STOP - Contraindicated 1
- If age ≥ 2 years: Proceed to Step 2
Step 2: Determine age-based dosing (FDA labeling uses age, not weight) 1
For children 2 to under 6 years:
- 1.25 to 2.5 mL every 4-6 hours
- Maximum: 10 mL in 24 hours
- Start at lower end of range (1.25 mL)
For children 6 to under 12 years:
- 2.5 to 5 mL every 4-6 hours
- Maximum: 20 mL in 24 hours
Critical Safety Considerations
Black Box Warning: Promethazine is contraindicated in children under 2 years due to risk of fatal respiratory depression 1. Even in children 2 years and older, use with extreme caution as respiratory depression remains a significant risk.
Lack of Efficacy: Research demonstrates that both promethazine and dextromethorphan are not superior to placebo for nocturnal cough in children with upper respiratory infections 2. The American Academy of Pediatrics does not support dextromethorphan use for acute cough in children 3.
Common Pitfalls to Avoid:
- Never calculate dose by weight alone - FDA dosing is age-based 1
- Do not assume a 13.79 kg child is automatically old enough for this medication
- Be aware that adverse effects occur more frequently with these medications compared to placebo 2
- Monitor for excessive sedation, respiratory depression, and paradoxical agitation 1
Drug Interactions
Absolute contraindications 1:
- MAO inhibitors (risk of hyperpyrexia, hypotension, death)
- Concurrent SSRIs (risk of serotonin syndrome) 4
Relative contraindications requiring dose reduction:
- Other CNS depressants, sedatives, narcotics, barbiturates
Practical Recommendation
Before prescribing, confirm the exact age in years and months. If there is any uncertainty about whether the child is 2 years or older, or if the child appears to be under 2 years based on developmental assessment, choose an alternative therapy. The evidence suggests that supportive care alone may be equally effective given the lack of superiority over placebo 2.
If the child is definitively 2-6 years old, start with 1.25 mL every 4-6 hours as needed, not to exceed 10 mL in 24 hours, and counsel caregivers extensively about respiratory depression risk and the need to avoid other sedating medications.