What is the efficacy of dextrometorphan (cough suppressant) compared to placebo in relieving cough symptoms in adults with a common cold?

Efficacy of Dextromethorphan for Cough in Common Cold

Dextromethorphan has modest efficacy in suppressing cough due to common cold in adults, but the generally recommended over-the-counter dosages are likely subtherapeutic, with maximum cough reflex suppression occurring at higher doses of 60 mg. 1

Mechanism and Evidence of Efficacy

• Dextromethorphan is a non-sedating opiate that acts as a cough suppressant by centrally modulating the cough reflex 1
• It has been shown to suppress acute cough in a single meta-analysis, supporting its use in common cold 1
• There is a clear dose-response relationship with dextromethorphan, with maximum cough reflex suppression occurring at 60 mg, which is higher than typical OTC doses 1
• Dextromethorphan is FDA-approved to temporarily relieve cough due to minor throat and bronchial irritation as may occur with the common cold 2
• Objective measures show dextromethorphan effectively diminishes cough reflex sensitivity in controlled studies, though subjective improvement may be similar to placebo 3

Clinical Application and Limitations

• Acute viral cough is almost invariably benign and prescribed treatment may be unnecessary in many cases 1
• Simple home remedies such as honey and lemon are recommended as first-line treatment before pharmacological options 4
• The British Thoracic Society recommends dextromethorphan as the preferred pharmacological option due to its better safety profile compared to other antitussives 4
• Dextromethorphan should be used with caution as some combination preparations contain additional ingredients like paracetamol 1
• Studies show mixed results regarding subjective improvement in cough symptoms, with some showing no significant difference between dextromethorphan and placebo 5, 6

Alternative Options

• Menthol by inhalation can provide acute but short-lived cough suppression 1
• First-generation antihistamines with sedative properties may suppress cough and are particularly useful for nocturnal cough 1, 4
• Codeine and pholcodine have no greater efficacy than dextromethorphan but have a much greater adverse side effect profile and are not recommended 1
• Combination products containing dextromethorphan with other ingredients (antihistamines, decongestants) may provide additional symptom relief for multiple cold symptoms 7, 8

Special Considerations

• The efficacy of dextromethorphan appears to be more consistently demonstrated in objective measures (cough challenge tests) than in subjective patient-reported outcomes 3
• A significant placebo effect is often observed in cough studies, which may explain the discrepancy between objective and subjective measures 3
• Care must be taken when recommending higher doses of dextromethorphan (where efficacy is better established) due to potential side effects and ingredients in combination products 1
• Voluntary suppression of cough may be sufficient to reduce cough frequency in some patients 1

Practical Approach

• For adults with bothersome cough from common cold, start with non-pharmacological approaches like honey and lemon 4
• If pharmacological treatment is needed, dextromethorphan is the preferred option, though higher doses (up to 60 mg) may be needed for optimal effect 1
• For nighttime cough disrupting sleep, consider first-generation antihistamines with sedative properties 1, 4
• Avoid codeine-containing products as they offer no advantage over dextromethorphan but have more side effects 1