What is the recommended dose of promethazine (Phenergan) for an adult patient with nausea?

Promethazine Dosing for Nausea in Adults

For nausea and vomiting in adults, administer promethazine 12.5-25 mg orally or intravenously every 4-6 hours as needed, with the FDA-approved standard dose being 25 mg. 1

Standard Dosing Recommendations

The FDA label establishes the following dosing parameters for nausea and vomiting 1:

• Initial dose: 25 mg (oral, IV, or rectal)
• Repeat dosing: 12.5-25 mg every 4-6 hours as needed
• Prophylactic dosing: 25 mg repeated at 4-6 hour intervals for prevention during surgery and postoperative period

Evidence-Based Lower Dose Options

Consider starting with 6.25-12.5 mg IV for comparable efficacy with reduced sedation, particularly when sedation is undesirable or when used with opioids 2, 3:

• 6.25 mg IV provides 74% relief at 1 hour, equivalent to ondansetron 4 mg, with significantly fewer adverse effects 2
• 12.5 mg IV achieves 68% relief at 1 hour with minimal difference in efficacy compared to 6.25 mg 2
• Lower doses (6.25 mg) result in less sedation at discharge compared to 12.5 mg, while maintaining equivalent antiemetic efficacy 3

Clinical Context and Route Selection

Oral/Rectal Administration

• Standard dose: 25 mg every 4-6 hours 1
• Reduced dose: 12.5 mg may be appropriate when sedation is problematic 1
• Oral bioavailability is approximately 25%, though FDA dosing does not differentiate by route 2

Intravenous Administration

• Infuse slowly to minimize hypotension risk 4
• Clinical effects evident within 5 minutes, duration 4-6 hours 4
• 12.5-25 mg IV is the recommended range for procedural sedation as adjunct to narcotics 4

Special Clinical Situations

Opioid-Induced Nausea

The NCCN guidelines specifically recommend promethazine 12.5-25 mg PO every 6 hours for patients with prior history of opioid-induced nausea 5:

• Use prophylactically in patients with known opioid-induced nausea 5
• Consider as breakthrough treatment for chemotherapy-induced nausea at 12.5-25 mg PO or IV every 4 hours 4

Cyclic Vomiting Syndrome

• 12.5-25 mg orally or rectally every 4-6 hours as abortive therapy during acute episodes 4

Elderly Patients

Start with 6.25 mg IV in patients ≥65 years 6:

• 6.25 mg is as effective as higher doses with significantly fewer adverse drug reactions (P = 0.048) 6
• No difference in time to relief between 6.25 mg and 12.5 mg groups 6

Critical Safety Considerations

Sedation Profile

• CNS depression and sedation occur frequently, which may be advantageous when sedation is desired but problematic otherwise 4
• Promethazine is more sedating than prochlorperazine, metoclopramide, or ondansetron 7
• Use cautiously when combined with opioids or other CNS depressants 4

Other Adverse Effects

• Hypotension with rapid IV administration 4
• Respiratory depression potential 4
• Extrapyramidal symptoms can develop, similar to other dopamine antagonists 4
• Dizziness, blurred vision, dry mouth 4

Contraindications

• Absolutely contraindicated in children under 2 years due to fatal respiratory depression risk 4, 1

Practical Algorithm for Dose Selection

1. Start with 12.5 mg IV/PO for most adult patients with nausea 1
2. Use 6.25 mg IV if patient is elderly (≥65 years), receiving concurrent opioids, or when sedation is undesirable 2, 6
3. Escalate to 25 mg if inadequate response after 4-6 hours 1
4. Maximum single dose: 25 mg 1
5. Repeat every 4-6 hours as needed 1

When Promethazine May Be Preferred

Promethazine is particularly useful when 4, 7:

• Sedation is beneficial (nighttime dosing, anxious patients)
• Other antiemetics have failed
• Cost is a consideration
• Opioid-induced pruritus is also present (dual benefit as antihistamine)