What is the dosing of promethazine (Phenergan) DM for coughing in an adult?

Promethazine DM Dosing for Cough in Adults

Promethazine DM is not a recommended first-line treatment for cough in adults, and you should instead prescribe dextromethorphan alone at 30-60 mg for optimal cough suppression, as promethazine offers no proven benefit for cough and carries significant sedation and other adverse effects. 1

Why Promethazine DM Should Be Avoided

The combination product "Promethazine DM" contains promethazine (an antihistamine) plus dextromethorphan. However, the evidence does not support using promethazine for cough management:

• Promethazine has no established efficacy for cough suppression and is primarily used for nausea, allergic conditions, and sedation 2, 3
• Promethazine causes significant sedation, agitation, hallucinations, seizures, and dystonic reactions, particularly concerning in vulnerable populations 3
• The sedative antihistamines mentioned in cough guidelines refer to first-generation agents for nocturnal cough, but promethazine specifically is not recommended for this indication 2

Recommended Alternative: Dextromethorphan Alone

For adult cough suppression, prescribe dextromethorphan 30-60 mg as a single dose or 10-15 mg three to four times daily (maximum 120 mg/day). 1

Optimal Dosing Strategy

• Maximum cough reflex suppression occurs at 60 mg of dextromethorphan, which is higher than standard over-the-counter preparations 1
• Standard OTC dosing (15-30 mg) is often subtherapeutic and may not provide adequate relief 1
• For regular dosing: 10-15 mg three to four times daily, with maximum daily dose of 120 mg 1

Important Safety Considerations

• Check for combination products containing acetaminophen when prescribing higher doses to avoid excessive acetaminophen exposure 1
• Dextromethorphan is absolutely contraindicated with MAOIs due to serotonin syndrome risk 4
• Use caution with SSRIs, SNRIs, TCAs, tramadol, meperidine, methadone, fentanyl, and amphetamines 4

Clinical Algorithm for Cough Management

First-Line Approach

• Start with non-pharmacological measures: honey and lemon mixtures may be as effective as pharmacological treatments for benign viral cough 1
• Consider voluntary cough suppression techniques 1

Second-Line Pharmacological Treatment

• Dextromethorphan 30-60 mg for additional relief if non-pharmacological measures fail 1
• For nocturnal cough disrupting sleep, consider first-generation sedating antihistamines (diphenhydramine 25-50 mg, NOT promethazine) 2
• Menthol inhalation provides quick but temporary relief 1

When to Reassess

• If cough persists beyond 3 weeks, discontinue antitussive therapy and perform full diagnostic workup for underlying causes 1
• For postinfectious cough, try inhaled ipratropium before central antitussives 1
• For severe paroxysms of postinfectious cough, consider prednisone 30-40 mg daily for a short course 1

Common Pitfalls to Avoid

• Do not prescribe promethazine-containing products for cough - there is no evidence of benefit and significant risk of adverse effects 3
• Do not use subtherapeutic doses of dextromethorphan (15-30 mg) expecting adequate relief 1
• Do not prescribe codeine-based antitussives - they have no greater efficacy than dextromethorphan but significantly more adverse effects including drowsiness, nausea, constipation, and physical dependence 2, 1
• Do not suppress productive cough where clearance of secretions is beneficial 1