Robitussin DM OTC Maximum Dose for Adults
For adults 12 years and older, the maximum dose of Robitussin DM (containing guaifenesin and dextromethorphan) is 20 mL every 4 hours for guaifenesin (not exceeding 6 doses in 24 hours) and 10 mL of dextromethorphan every 12 hours (not exceeding 20 mL in 24 hours). 1, 2
Specific Dosing Parameters
Guaifenesin Component
- Standard adult dose: 10-20 mL (2-4 teaspoonfuls) every 4 hours 1
- Maximum frequency: Do not exceed 6 doses in any 24-hour period 1
- Maximum daily amount: 120 mL (24 teaspoonfuls) in 24 hours 1
Dextromethorphan Component
- Standard adult dose: 10 mL every 12 hours 2
- Maximum daily amount: 20 mL in 24 hours 2
- Important note: Shake bottle well before use and measure only with the dosing cup provided 2
Clinical Efficacy Considerations
For optimal cough suppression, dextromethorphan demonstrates maximum efficacy at 60 mg doses, which is higher than many OTC formulations provide. 3 This means standard OTC dosing may be subtherapeutic for severe cough 3. The dose-response relationship shows that commonly prescribed doses are often inadequate for maximum cough suppression 3.
When Dextromethorphan is Most Appropriate
- Dextromethorphan is the preferred pharmacological agent for dry cough due to superior safety compared to codeine-based alternatives 3
- It is a non-sedating opiate that effectively suppresses the cough reflex 3
- First-generation sedating antihistamines may be more appropriate for nocturnal cough when sleep disruption is the primary concern 3
Critical Safety Warnings
Do Not Exceed Maximum Doses
- Megadoses (5-10 times recommended dose) can cause profound psychological and physiological effects similar to phencyclidine (PCP) 4
- Approximately 5% of persons of European ethnicity lack the ability to metabolize dextromethorphan normally, leading to rapid toxic levels 4
- Overdose toxicity is additive when combined with other ingredients like acetaminophen, chlorpheniramine, or pseudoephedrine 4
When NOT to Use Cough Suppressants
- Do not use in patients requiring assessment for pneumonia (characterized by tachycardia, tachypnea, fever, or abnormal chest examination) 3
- Avoid suppressing productive cough in conditions like pneumonia or bronchiectasis where mucus clearance is essential 3
- In patients with asthma or COPD, treat the underlying disease first rather than suppressing cough if it serves a protective clearance function 3
Red Flags Requiring Immediate Medical Evaluation
- Hemoptysis (coughing up blood) 3
- Increasing breathlessness or tachypnea 3
- Fever with purulent sputum suggesting serious lung infection 3
- Cough persisting beyond 8 weeks 3
Alternative First-Line Approaches
Simple home remedies like honey and lemon are recommended as first-line treatment before pharmacological intervention, as they are the simplest, cheapest, and often effective option. 3 Voluntary cough suppression through central modulation may be sufficient to reduce cough frequency 3.