Robitussin DM (Dextromethorphan/Guaifenesin): Comprehensive Medication Profile
Classification
Dextromethorphan is the preferred first-line antitussive agent for dry cough, while guaifenesin functions as an expectorant with limited evidence for efficacy. 1
- Dextromethorphan: Non-opioid, centrally-acting antitussive (cough suppressant) 1, 2
- Guaifenesin: Expectorant agent intended to loosen respiratory secretions 3
Mechanism of Action
- Dextromethorphan acts centrally to suppress the cough reflex through modulation of the medullary cough center, functioning as a non-sedating opiate analog without analgesic or addictive properties at therapeutic doses 1, 2, 4
- Guaifenesin theoretically increases respiratory tract fluid secretions to reduce mucus viscosity and facilitate expectoration, though the exact mechanism remains incompletely understood 3
Effects on the Heart
- No clinically significant direct cardiac effects are documented for either dextromethorphan or guaifenesin at therapeutic doses 1, 2
Effects on Other Body Systems
Dextromethorphan
- Central Nervous System: At therapeutic doses, produces cough suppression without sedation, analgesia, or respiratory depression characteristic of opioids 4
- At supratherapeutic doses (megadoses): Can produce dissociative effects similar to phencyclidine (PCP), including profound psychological alterations, which represents a significant abuse risk in adolescents 5, 6
- Neuroprotective properties: Demonstrates anticonvulsant and cerebral ischemia protection in experimental models 4
Guaifenesin
- Respiratory tract: Increases sputum volume and decreases viscosity over 4-6 days in productive cough 3
- Limited systemic effects at recommended doses 3
Indications
Evidence-Based Indications
- Dextromethorphan: Symptomatic relief of non-productive (dry) cough associated with upper respiratory infections, common cold, and acute bronchitis 1, 2, 7
- Guaifenesin: Productive cough with thick, tenacious mucus requiring facilitation of expectoration 3
Critical Caveat
- Guaifenesin is NOT indicated for dry cough and provides no cough-suppressive benefit; it is inappropriate when the therapeutic goal is cough suppression rather than secretion clearance 2
- Insufficient evidence supports guaifenesin efficacy as an adjunct in sinusitis or for cough suppression 3
Route and Dosage
Dextromethorphan
Standard over-the-counter dosing is subtherapeutic; optimal cough suppression requires higher doses. 1, 2
- Suboptimal OTC dose: 10-15 mg every 6-8 hours 1
- Optimal therapeutic dose: 30-60 mg every 6-8 hours for maximal cough reflex suppression 1, 2
- Maximum daily dose: 120 mg 1
- Bedtime dosing: 15-30 mg to suppress nocturnal cough and promote sleep 1
- Route: Oral (liquid, tablet, capsule formulations) 1
Guaifenesin
- Standard dose: 200-400 mg every 4 hours as needed 8
- Maximum daily dose: Typically 2400 mg 8
- Route: Oral 8
Titration Parameters
- Dextromethorphan: Dose escalation from 30 mg to 60 mg based on inadequate cough suppression; no further benefit beyond 60 mg per dose 1, 2
- Guaifenesin: No specific titration required; use standard dosing intervals 8
- Clinical endpoint: Reduction in cough frequency, severity, and improvement in sleep quality 1, 2
Contraindications and Precautions
Absolute Contraindications
- Do NOT use dextromethorphan in patients requiring pneumonia assessment (tachycardia, tachypnea, fever, abnormal chest examination) until pneumonia is ruled out 2
- Avoid in productive cough where secretion clearance is essential (e.g., pneumonia, bronchiectasis, COPD with significant sputum production) 2
Precautions
- Combination products: Exercise extreme caution with higher dextromethorphan doses in products containing acetaminophen or other active ingredients to avoid toxic accumulation 1, 2
- CYP2D6 poor metabolizers (~5% of European populations) may experience rapid toxic accumulation even at standard doses 5
- Abuse potential: Adolescents and young adults may misuse dextromethorphan for dissociative effects; monitor for recreational use 5, 6
- Drug interactions: Avoid concurrent use with MAO inhibitors (risk of serotonin syndrome) 5
- Renal impairment: No dose adjustment required for dextromethorphan (hepatically metabolized via CYP2D6) 1
Guaifenesin-Specific
- Not appropriate for dry cough; using it in this context represents a prescribing error 2
Hold Parameters
- Hold dextromethorphan if:
- Signs of pneumonia develop (fever, tachypnea, tachycardia, abnormal lung sounds) 2
- Hemoptysis occurs 2
- Cough becomes productive with purulent sputum requiring clearance 2
- Neurological symptoms suggesting toxicity appear (confusion, ataxia, visual disturbances) 5
- Cough persists beyond 3 weeks without improvement (requires diagnostic workup) 1
Adverse Reactions and Side Effects
Dextromethorphan (Therapeutic Doses)
- Generally well-tolerated with superior safety profile compared to codeine 1, 2, 9
- Minimal side effects: Mild dizziness, gastrointestinal upset (rare) 1
- No physical dependence at therapeutic doses 1, 9
- No respiratory depression unlike opioid antitussives 4, 9
Dextromethorphan (Overdose/Abuse)
- Dissociative effects: Hallucinations, altered perception, euphoria similar to PCP 5, 6
- Neurological: Ataxia, confusion, nystagmus, slurred speech 5
- Cardiovascular: Tachycardia, hypertension 5
- False-positive urine drug screen for phencyclidine (PCP) 5
- Death reported with massive overdose 6
Guaifenesin
Drug Interactions
Dextromethorphan
- MAO inhibitors: Contraindicated due to risk of serotonin syndrome (hyperthermia, rigidity, autonomic instability) 5
- SSRIs/SNRIs: Theoretical risk of serotonin syndrome at high doses; use caution 5
- CYP2D6 inhibitors (e.g., quinidine, fluoxetine, paroxetine): May increase dextromethorphan levels and toxicity risk 5
Guaifenesin
- Minimal clinically significant interactions documented 8
Assessment, Monitoring, and Desired Outcomes
Pre-Treatment Assessment
- Characterize cough: Dry vs. productive, duration (<3 weeks acute, 3-8 weeks subacute, >8 weeks chronic), severity, impact on sleep 2
- Rule out red flags: Hemoptysis, fever with purulent sputum, tachypnea, abnormal chest examination, progressive breathlessness 2, 7
- Medication history: Identify combination products to avoid ingredient duplication (especially acetaminophen) 1, 2
- Screen for abuse risk: Particularly in adolescents and young adults 5, 6
Monitoring Parameters
- Cough frequency and severity: Should decrease within 24-48 hours of optimal dextromethorphan dosing 1, 2
- Sleep quality: Improvement in nocturnal cough and sleep disruption 1
- Duration of use: Discontinue if no improvement after 7 days or if cough persists beyond 3 weeks (requires diagnostic evaluation) 1, 2
- Signs of toxicity: Monitor for CNS effects, especially in first-time users or with dose escalation 5
Desired Outcomes
- Primary: Reduction in cough frequency by 40-60% in chronic bronchitis; less robust effect (≤20%) in acute URI 2
- Secondary: Improved sleep quality, reduced cough-related quality of life impairment 2
- Realistic expectation: Dextromethorphan provides modest symptomatic relief but does not eliminate cough entirely; effect is most pronounced at 60 mg dosing 1, 2
Treatment Failure Management
- If inadequate response to dextromethorphan 60 mg after 3-5 days:
Critical Clinical Pearls
- Codeine offers NO advantage over dextromethorphan but significantly increases adverse effects (drowsiness, constipation, dependence risk); it should not be prescribed 1, 2, 9
- Honey and lemon are as effective as pharmacological treatments for benign viral cough and should be recommended first-line 1, 2, 7
- Guaifenesin has Grade D evidence (good evidence of no benefit) for cough suppression in chronic bronchitis and should not be used for dry cough 2
- Combination dextromethorphan/guaifenesin products are irrational for dry cough; the guaifenesin component provides no therapeutic benefit in this context 2
- Most acute viral cough is self-limiting (1-3 weeks); reassurance and symptomatic management are appropriate 2, 7