Proper Dosage and Usage of Promethazine for Nausea and Sedation
For nausea and vomiting in adults, the recommended dosage of promethazine is 12.5-25 mg administered orally, intravenously (infused slowly at 25 mg/min), or rectally every 4-6 hours as needed, with a maximum total dose of 25-50 mg when used as an adjuvant to narcotics and benzodiazepines. 1, 2
Dosage for Nausea and Vomiting
Adults
- For oral administration: 25 mg as a single dose, or 12.5 mg taken before meals and at bedtime 2
- For intravenous administration: 12.5-25 mg infused slowly (25 mg/min) to minimize hypotension risk 1, 3
- Duration of action is 4-6 hours with clinical effects evident within 5 minutes of IV administration 1, 3
- For prophylaxis of nausea and vomiting (e.g., during surgery): 25 mg repeated at 4-6 hour intervals 2
- Lower doses (6.25 mg IV) have been shown to be as effective as ondansetron 4 mg for treating nausea and vomiting 4
Children
- For children over 2 years: 0.5 mg per pound of body weight 2
- Promethazine is contraindicated in children under 2 years of age 2, 5
Dosage for Sedation
Adults
- For nighttime sedation: 25-50 mg orally 2
- For pre-operative sedation: 50 mg the night before surgery 2
- For post-operative sedation: 25-50 mg 2
Children
- For children over 2 years: 12.5-25 mg orally or rectally at bedtime 2
- For pre-operative sedation: 0.5 mg per pound of body weight in combination with reduced doses of narcotics or barbiturates 2
Administration Considerations
- When administered intravenously, promethazine should be infused slowly (25 mg/min) to minimize the risk of hypotension 1, 3
- Intramuscular administration is preferred over intravenous to reduce risk of serious adverse effects 6
- When used as an adjuvant to narcotics and benzodiazepines, promethazine may require a reduction in the dosage of these standard sedation agents 1
- The clinical effects of promethazine are evident within 5 minutes of intravenous administration with a duration of action of 4-6 hours 1, 3
- The plasma half-life of promethazine is 9-16 hours 1, 3
Mechanism of Action
- Promethazine is a phenothiazine that possesses antihistamine, sedative, antiemetic, and anticholinergic effects 1, 3
- It blocks postsynaptic dopaminergic receptors in the brain and has strong alpha-adrenergic inhibitory effects 3
- It competitively blocks histamine H1 receptors, inhibiting most pharmacologic effects of histamine 3
Important Adverse Effects and Precautions
- Hypotension may occur, particularly with rapid IV administration 1, 3
- Respiratory depression can occur, especially when combined with other CNS depressants 3, 7
- Extrapyramidal effects ranging from restlessness to oculogyric crises may occur 1, 7
- Serious tissue injury can result from extravasation, inadvertent intra-arterial injection, or intraneuronal infiltration 1, 6
- Enhanced sedative effects occur when combined with alcohol, benzodiazepines, or opioids 3
- The anticholinergic effects may cause dry mouth, blurred vision, urinary retention, and wheezing 1
Clinical Pearls
- Lower doses (6.25-12.5 mg) of promethazine may be as effective as standard doses with fewer side effects 4
- When used as an adjuvant to narcotics and benzodiazepines for sedation, promethazine may allow for reduced dosing of these agents 1
- The FDA has issued warnings regarding the risks of tissue injury with injectable promethazine, recommending careful administration techniques 6, 7
- Alternative antiemetics like ondansetron may be preferred in some settings due to their more favorable side effect profile 7