Maximum Daily Doses of Dextromethorphan and Guaifenesin
The maximum daily dose of dextromethorphan is 120 mg for adults and children 12 years and older, while the maximum daily dose of guaifenesin is 2400 mg (6 doses of 400 mg every 4 hours) for adults and children 12 years and older. 1, 2
Dextromethorphan Maximum Daily Doses
According to the FDA drug label, dextromethorphan dosing follows this pattern:
Adults and children 12 years and older: 10 mL every 12 hours, not to exceed 20 mL in 24 hours 1
- This typically corresponds to 30 mg per dose or 60 mg per day in standard formulations
- However, the absolute maximum daily dose is 120 mg 3
Children 6 to under 12 years: 5 mL every 12 hours, not to exceed 10 mL in 24 hours 1
- This typically corresponds to 15 mg per dose or 30 mg per day
Children 4 to under 6 years: 2.5 mL every 12 hours, not to exceed 5 mL in 24 hours 1
- This typically corresponds to 7.5 mg per dose or 15 mg per day
Children under 4 years: Not recommended 1
Guaifenesin Maximum Daily Doses
According to the FDA drug label, guaifenesin dosing follows this pattern:
Adults and children 12 years and older: 10 to 20 mL (2 to 4 teaspoonfuls) every 4 hours 2
- This typically corresponds to 200-400 mg per dose
- Maximum of 6 doses in any 24-hour period 2
- Therefore, maximum daily dose is 2400 mg (6 × 400 mg)
Children 6 to under 12 years: 5 to 10 mL (1 to 2 teaspoonfuls) every 4 hours 2
- This typically corresponds to 100-200 mg per dose
- Maximum of 6 doses in any 24-hour period 2
- Therefore, maximum daily dose is 1200 mg (6 × 200 mg)
Children 2 to under 6 years: 2.5 to 5 mL (½ to 1 teaspoonful) every 4 hours 2
- This typically corresponds to 50-100 mg per dose
- Maximum of 6 doses in any 24-hour period 2
- Therefore, maximum daily dose is 600 mg (6 × 100 mg)
Children under 2 years: Consult a physician 2
Clinical Considerations
Safety Concerns with Dextromethorphan
- Dextromethorphan has potential for abuse, with adolescents sometimes taking "megadoses" (5-10 times the recommended dose) to achieve dissociative effects 4
- At therapeutic doses, dextromethorphan does not appear to impair driving ability 3
- Approximately 5% of people of European ethnicity lack the ability to metabolize dextromethorphan normally, which can lead to rapid acute toxic levels even with moderate overdoses 4
Efficacy Considerations
- Evidence for dextromethorphan's effectiveness in adults shows modest decrease in cough compared to placebo 5
- In children, dextromethorphan does not appear to relieve cough better than placebo 5
- Similarly, guaifenesin has not been shown to provide greater relief than placebo in adults 5
Drug Interactions
- Dextromethorphan can interact with probenecid and other drugs that inhibit renal tubular secretion, leading to increased plasma concentrations 6
- When used with other serotonergic medications, dextromethorphan may increase the risk of serotonin syndrome 6
- Patients with poor CYP2D6 metabolizer status, renal impairment, or those taking CYP2D6 inhibitors like quinidine may experience higher plasma concentrations of dextromethorphan 7
Remember that these maximum doses are for short-term symptomatic relief. If symptoms persist beyond a few days, patients should be evaluated for underlying conditions requiring different treatment approaches.