Dextromethorphan Syrup Prescribing Guide
Recommended Dosing
For acute cough in adults and children, dextromethorphan should be prescribed at higher doses than standard over-the-counter recommendations to achieve effective cough suppression, with 60 mg providing optimal antitussive effect in adults. 1
Adult Dosing (≥12 years)
- Standard dose: 10 mL every 12 hours (not to exceed 20 mL in 24 hours) 2
- Optimal therapeutic dose: 60 mg for maximum cough reflex suppression 1, 3
- Standard OTC dosing is often subtherapeutic; maximum suppression occurs at 60 mg 1
Pediatric Dosing
- Ages 6 to <12 years: 5 mL every 12 hours (maximum 10 mL in 24 hours) 2
- Ages 4 to <6 years: 2.5 mL every 12 hours (maximum 5 mL in 24 hours) 2
- Under 4 years: Do not use 2
- Weight-based dosing of 0.5 mg/kg may provide better symptom control while minimizing adverse events 4
Clinical Algorithm for Prescribing
Step 1: Assess Cough Type
- Dry, non-productive cough: Dextromethorphan is appropriate 1
- Productive cough: Do not suppress; consider expectorants instead 5
Step 2: First-Line Approach
- Start with non-pharmacological measures (honey and lemon) for benign viral cough 1, 3
- Consider voluntary cough suppression techniques 1
Step 3: Pharmacological Treatment
- If non-pharmacological measures fail, prescribe dextromethorphan 1
- Use appropriate therapeutic doses (30-60 mg in adults) rather than standard OTC doses 1, 3
- For nocturnal cough disrupting sleep, consider first-generation antihistamines instead 1, 3
Step 4: Duration and Monitoring
- Prescribe for short-term use only 3
- Reassess if cough persists beyond 2-3 weeks to evaluate for underlying causes 5
Critical Prescribing Considerations
Dosing Pitfalls to Avoid
- Do not underdose: Standard OTC preparations often contain subtherapeutic amounts 1
- Check combination products: Some formulations contain acetaminophen or other ingredients that limit maximum safe dosing 1
- Avoid codeine alternatives: Codeine has no greater efficacy than dextromethorphan but significantly more adverse effects (drowsiness, nausea, constipation, dependence) 1, 3
Safety Warnings
- Abuse potential: At doses >1500 mg/day, dextromethorphan can induce PCP-like psychosis with delusions, hallucinations, and paranoia 6
- Not detected on standard urine drug screens: Consider in differential for substance-induced psychosis 6
- Shake bottle well before use and measure only with provided dosing cup 2
Evidence Quality and Nuances
The evidence for dextromethorphan efficacy is mixed. While guidelines recommend it as first-line due to superior safety profile 1, 3, research shows inconsistent results. A 2000 study found minimal support for 30 mg doses 7, while a 2023 pediatric study demonstrated 21-25% reduction in cough frequency with proper dosing 8. The key insight is that dose-response relationships exist, with 60 mg providing maximum suppression 1, 3, explaining why standard OTC doses often fail.
When Dextromethorphan May Not Work
- Postinfectious cough: Try inhaled ipratropium first 1
- Severe paroxysms: Consider short-course prednisone 30-40 mg daily 1
- Suspected pertussis: Macrolide antibiotics indicated 1