What is the recommended dosage for promethazine (Phenergan) cough syrup?

Promethazine Cough Syrup Dosage

Promethazine is not recommended for cough suppression and should not be used for this indication. 1, 2

Why Promethazine Should Not Be Used for Cough

• Promethazine has no established efficacy for cough suppression and is primarily indicated for nausea, allergic conditions, and sedation—not cough management 1
• The British Thoracic Society explicitly recommends against using promethazine for cough, as it is not an appropriate indication for this medication 1
• Dextromethorphan is the recommended first-line antitussive agent due to its superior safety profile and proven efficacy for cough suppression 1

Recommended Alternative: Dextromethorphan

First-Line Approach

• Start with simple non-pharmacological measures like honey (one teaspoon) for benign viral cough 3, 1
• If pharmacological treatment is needed, use dextromethorphan 30-60 mg orally for optimal cough suppression 1
• Standard over-the-counter dextromethorphan dosing (10-15 mg) is often subtherapeutic; maximum cough reflex suppression occurs at 60 mg 1

Dosing Algorithm for Dextromethorphan

• Initial dose: 30 mg orally every 4-6 hours as needed 1
• Maximum single dose: 60 mg for severe cough requiring maximum suppression 1
• Maximum daily dose: 120 mg 1
• For nocturnal cough disrupting sleep, consider first-generation sedating antihistamines instead 1

If Promethazine Is Mistakenly Prescribed (FDA-Approved Indications Only)

Critical Safety Warning

• Promethazine is contraindicated in children under 2 years of age due to risk of fatal respiratory depression 4

Standard FDA-Approved Dosing (Not for Cough)

For nausea/vomiting only (the actual FDA indication):

• Adults: 12.5-25 mg orally every 4-6 hours as needed 4
• Children ≥2 years: 0.5 mg per pound of body weight, adjusted to age and severity 4
• Doses may be repeated at 4-6 hour intervals as necessary 4

Important Safety Considerations

• Lower doses (6.25-12.5 mg) are equally effective for antiemetic purposes and cause significantly less sedation 2, 5
• Significant sedation occurs with promethazine, particularly when combined with opioids or other CNS depressants 2
• Extrapyramidal effects including dystonia and neuroleptic malignant syndrome can occur, making it inappropriate for chronic use 2
• Avoid IV administration when possible due to risks of tissue necrosis, thrombophlebitis, and gangrene 2
• Onset of action is within 20 minutes orally, with duration of 4-6 hours (effects may persist up to 12 hours) 2

Common Pitfalls to Avoid

• Do not prescribe promethazine for cough—it lacks efficacy for this indication and carries unnecessary risks 1, 2
• Do not use subtherapeutic doses of dextromethorphan (10-15 mg) when 30-60 mg is needed for adequate cough suppression 1
• Do not prescribe codeine-based antitussives, which have no greater efficacy than dextromethorphan but significantly more adverse effects (drowsiness, nausea, constipation, physical dependence) 3, 1
• Do not use promethazine in children under 2 years of age under any circumstances 4