Is it appropriate to prescribe 150mg promethazine (Phenergan) suppositories?

Prescribing 150mg Promethazine Suppositories

Prescribing 150mg promethazine suppositories is not appropriate as this exceeds the recommended dosage range of 12.5-25mg for intravenous use or 25mg for suppository use. 1, 2

Recommended Dosing for Promethazine

• The usual dose of promethazine is 12.5-25mg when administered intravenously, infused slowly (25 mg/min) to minimize the risk of hypotension 1, 2
• For suppository use, the standard dose is 25mg, as evidenced by clinical studies evaluating efficacy in outpatient surgical patients 3
• A total dose of 25-50mg may be used as an adjuvant to narcotics and benzodiazepines for procedures such as endoscopy, but this represents the maximum recommended dose 1

Safety Concerns with Higher Doses

• Promethazine can cause significant adverse effects even at standard doses, including:

  • Respiratory depression, particularly when combined with other CNS depressants 2
  • Hypotension, especially with rapid administration 2
  • Extrapyramidal effects ranging from restlessness to oculogyric crises 1
  • Neuroleptic malignant syndrome 1
  • Sedation, which may be excessive at higher doses 4

• Higher doses (such as 150mg) would substantially increase these risks without providing additional therapeutic benefit 2, 5

Appropriate Clinical Use of Promethazine

• Promethazine is indicated for:

  • Management of postoperative nausea and vomiting 2
  • Treatment of allergic reactions 2
  • Adjuvant for postoperative pain and sedation 2

• When using promethazine suppositories, the 25mg strength has been shown to be effective for post-discharge nausea and vomiting in outpatient surgical patients 3

Alternative Antiemetic Options

• Ondansetron may be preferred over promethazine in many clinical scenarios due to:
  • Favorable safety profile with fewer CNS and autonomic side effects 5
  • Efficacy comparable to promethazine without associated sedation or extrapyramidal effects 4
  • Availability in multiple formulations, including orally disintegrating tablets 5

Key Clinical Considerations

• Promethazine exhibits enhanced sedative effects when combined with other CNS depressants such as benzodiazepines and opioids, making dose control critical 2
• The FDA has issued warnings regarding injectable promethazine due to risks of extravasation and tissue necrosis 6, 5
• Pediatric use requires special caution, with reports of significant sedation, agitation, hallucinations, seizures, and dystonic reactions 7

Remember that exceeding recommended dosages increases risk without providing additional therapeutic benefit. The 150mg dose is six times the standard suppository dose and three times the maximum recommended total dose for any route of administration.