Phenergan (Promethazine) Suppository Dosing Frequency
For nausea and vomiting, promethazine suppositories can be given every 4-6 hours as needed in both adults and children over 2 years of age, with doses of 12.5-25 mg per administration. 1
Standard Dosing Intervals
The FDA-approved dosing for promethazine suppositories allows repeat administration every 4-6 hours as necessary for nausea and vomiting. 1 This applies to both oral and rectal formulations, with the specific guidance that 12.5-25 mg doses may be repeated at these intervals. 1
For pediatric patients specifically:
- Children require 0.5 mg per pound of body weight (approximately 1 mg/kg), adjusted to age, weight, and severity of condition 1
- The same 4-6 hour interval applies to pediatric dosing 1
- Promethazine is contraindicated in children under 2 years of age due to risk of respiratory depression 1
Clinical Context and Practical Considerations
The 4-6 hour dosing interval for antiemetic use is consistent across multiple clinical guidelines. 2 In the management of cyclic vomiting syndrome, promethazine 12.5-25 mg is recommended every 4-6 hours during episodes, which aligns with the FDA labeling. 2
Important caveats about rectal administration:
- Rectal formulations have erratic absorption compared to oral routes 3
- Some patients may receive inadequate relief while others risk drug accumulation with repeated dosing 3
- Oral syrup is preferred when possible due to more consistent absorption 3
Safety Monitoring
The sedation rate with promethazine is 60-73%, substantially higher than second-generation antihistamines. 3 This high sedation rate means patients should be monitored for:
- Excessive drowsiness that could impair function
- Anticholinergic effects (dry mouth, urinary retention, confusion)
- Extrapyramidal symptoms or dystonic reactions 2
If acute dystonic reactions occur, administer diphenhydramine 1-2 mg/kg or 25-50 mg parenterally. 3
Maximum Dosing Limits
While the FDA label specifies the 4-6 hour interval, it does not explicitly state a maximum number of doses per 24 hours. 1 However, the standard practice based on the 4-6 hour interval would allow for 4-6 doses maximum in a 24-hour period (every 6 hours = 4 doses; every 4 hours = 6 doses).
For prophylaxis of postoperative nausea and vomiting, the average dose is 25 mg repeated at 4-6 hour intervals as necessary. 1