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Background: Limitations in antiemetic options have resulted in increased use of intravenous promethazine. However, this drug has significant sedative effects with its standard dosage of 25 mg, especially when used in conjunction with narcotic analgesics. While studies have revealed the bioavailability of enteric promethazine to be 25%, current dosing references suggest identical dosing regardless of the route of administration. Objective: To compare the antiemetic efficacy and sedative effects of low-dose intravenous promethazine (6.25 or 12.5 mg) with intravenous ondansetron 4 mg. Methods: We assessed inpatients with noncritical conditions at Anne Arundel Medical Center who were treated for nausea or vomiting from any cause except chemotherapy or pregnancy. Forty-six patients received low-dose promethazine and 41 received ondansetron. Statistical analysis was carried out for significant differences in efficacy and sedation. Results: For patients who received intravenous promethazine 6.25 or 12.5 mg, nausea and vomiting were relieved at one hour in 74% and 68%, respectively, compared with 59% for intravenous ondansetron 4 mg. Results at 3 hours were 67% and 80% for promethazine and 71% for ondansetron. Median sedation scores at one hour were equal at 3 for promethazine and ondansetron (4 = fully awake); at 3 hours, the median scores were 4 and 3.5, respectively. There were no statistically significant differences among any of these results. Conclusions: Low-dose (6.25 mg) intravenous promethazine relieves nausea and vomiting as effectively as intravenous ondansetron 4 mg.