What is the recommended dose of dextromethorphan (cough suppressant) for a 19-year-old?

Dextromethorphan Dosing for a 19-Year-Old

For a 19-year-old with cough, the recommended dose of dextromethorphan is 10 mL (30 mg) every 12 hours, not to exceed 20 mL (60 mg) in 24 hours, though evidence suggests 60 mg provides optimal cough suppression. 1

Standard FDA-Approved Dosing

For adults and children 12 years of age and over:

• 10 mL every 12 hours (typically 30 mg per dose)
• Maximum: 20 mL (60 mg) in 24 hours 1

Evidence-Based Optimal Dosing

The FDA-approved dosing may be subtherapeutic for maximal effect:

• Standard over-the-counter dosing is often inadequate for optimal cough suppression 2
• Maximum cough reflex suppression occurs at 60 mg and can provide prolonged relief 2
• Dextromethorphan 60 mg provides optimal cough suppression compared to lower doses 2
• The 30 mg dose may provide modest reduction in cough (19-36%), but higher doses show better efficacy 2

Clinical Approach

For symptomatic relief of non-productive cough:

• Start with 30 mg every 12 hours as per FDA labeling 1
• If inadequate relief, may increase to 60 mg per dose (not exceeding 60 mg in 24 hours for twice-daily dosing) 2
• Consider non-pharmacological approaches first (honey and lemon) as they may be equally effective 2

For nocturnal cough specifically:

• Consider first-generation sedating antihistamines as an alternative, particularly suitable for nighttime symptoms 2

Important Safety Considerations

Avoid codeine-based alternatives:

• Codeine has no greater efficacy than dextromethorphan but carries a worse adverse effect profile (drowsiness, nausea, constipation, physical dependence) 2

Caution with combination products:

• Many dextromethorphan preparations contain additional ingredients like acetaminophen, which can cause toxicity at higher doses 2, 3
• Always check the formulation to avoid inadvertent overdose of other components

Toxicity threshold:

• Doses exceeding 7.5 mg/kg (approximately 525 mg for a 70 kg adult) require emergency department evaluation 4
• For a 19-year-old, therapeutic doses of 30-60 mg are well below this threshold

Drug interactions:

• If the patient is taking monoamine oxidase inhibitors or selective serotonin reuptake inhibitors, there is risk of serotonin syndrome 4
• Approximately 5% of persons of European ethnicity lack normal metabolism capacity, leading to potentially toxic levels even at standard doses 3

When Dextromethorphan Should NOT Be Used

• Productive cough where clearance of secretions is beneficial 2
• Postinfectious cough: Try inhaled ipratropium first; use dextromethorphan only when other measures fail 2
• Suspected pertussis: Macrolide antibiotics are indicated instead 2