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Introduction
The 5-HT3 receptor antagonist ondansetron is used to prevent and treat postoperative nausea and vomiting. While writing a systematic review of ondansetron's efficacy in treating established postoperative nausea and vomiting we found that one large multicentre trial had been published three times. 1
Duplicate publication may be overt, such as reanalysis of an important trial with appropriate cross referencing to original reports, or it may be covert, with the same data published again without cross referencing, often with no intention to provide novel information. Covert duplication may result in qualitative exaggeration of an intervention's efficacy, 2 3 but it also poses a threat to quantitative (meta) analysis. The danger is that data from the same patient will be analysed more than once, leading to biased estimates of treatment efficacy, exaggerated accuracy, and a false impression of drug safety.
We therefore searched for all published full reports of randomised controlled trials of ondansetron in the surgical setting to identify duplicated trials (both covert and overt) and efficacy data and to measure the impact of any duplication on the estimate of ondansetron's efficacy.
Methods
Systematic search
A systematic search was performed for published full reports of randomised controlled trials which tested the antiemetic efficacy of prophylactic and therapeutic ondansetron compared with placebo, no treatment, or other antiemetics on nausea and vomiting after a general anaesthetic. Medline (Knowledge Finder 4.0, Silver Platter 3.25), Embase, and Biological Abstracts databases were searched, without restriction to the English language and using different search strategies 1 4 with free term combinations (date of the last electronic search 19 September 1996). Additional reports were identified from reference lists of retrieved reports and review articles on postoperative emesis and ondansetron and by hand searching locally available anaesthesia journals. We checked our database with the database of published trials provided by the manufacturer of ondansetron. Abstracts were not considered. Unpublished trials were not sought. Authors of reports were contacted to clarify uncertainty about duplicate publications. If there was an ambiguous reply or no reply we contacted the manufacturer.
Extraction of data
Information on authors, journals (parent journal or supplement), number of patients per treatment and control arm, sponsorship, cross references to reports with similar data or preliminary reports, and anaesthetic techniques was taken from...