Objectives: To test the evidence for a dose-response with ondansetron for treatment of postoperative nausea and vomiting and to establish whether differences in efficacy between doses are of clinical relevance.

Design: Quantitative systematic review of published randomised controlled trials.

Data sources: Seven trials from 1991 to January 1996 retrieved from a systematic literature search (Medline, reference lists, hand searching of anaesthetic journals, manufacturer9s database); no restriction on language.

Main outcome measures: Estimation of efficacy (incidence of complete control of further nausea and vomiting) by using odds ratios and the “number needed to treat” method for early (within 6 hours of administration) and late (within 24 hours) periods.

Results: Four placebo controlled trials with 1043 patients studied intravenous ondansetron 1 mg, 4 mg, or 8 mg. All doses were more efficacious than placebo in preventing further episodes of nausea or vomiting. For combined data, the point estimates for the number needed to treat were between 3.1 (8 mg) and 3.8 (1 mg) for early efficacy and between 4.1 (8 mg) and 4.8 (1 mg) for late efficacy, without significant differences between doses. No difference was found between ondansetron and droperidol in two trials with 129 patients or between ondansetron and metoclopramide in one trial with 80 patients.

Conclusions: Further nausea and vomiting could be prevented with ondansetron compared with placebo in 25% of patients who had nausea or vomiting (number needed to treat, about 4). There was no evidence of a clinically relevant dose-response between 1 mg and 8 mg or a difference between ondansetron and either droperidol or metoclopramide in a limited dataset. A false impression of ondansetron9s efficacy may arise because a quarter of all relevant published reports are duplicates, and reporting of study results is uncritical.

Little information exists on the efficacy of anti-emetic interventions in patients with established postoperative nausea and vomiting

To evaluate the effectiveness of ondansetron in this setting we conducted a quantitative systematic review of all relevant published randomised controlled trials

Four trials (1043 patients) compared intravenous ondansetron 1 mg, 4 mg, or 8 mg with placebo, two trials (129 patients) compared ondansetron with droperidol, and one trial (80 patients) compared ondansetron with metoclopramide

All three tested doses of ondansetron were more efficacious than placebo. There was no evidence of a clinically relevant dose-response between 1 mg and 8 mg (number needed to treat to prevent further nausea or vomiting was about 4), or a difference between ondansetron and either droperidol or metoclopramide.

Stopping further postoperative nausea and vomiting in 25% of the patients may be the best that can be achieved currently