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Introduction:Orlistat, the first of a new class of drugs for the treatment of obesity, was launched in the UK in December 1998. The prescribing information recommends that treatment with orlistat should be discontinued after 12 weeks if the patient has not achieved a specified loss of weight.Objective:To monitor the safety of orlistat prescribed in the primary care setting in England using prescription-event monitoring (PEM).Methods:A postmarketing surveillance study using the observational cohort technique of PEM. Patients were identified from dispensed prescriptions issued by primary care physicians for orlistat between December 1998 and November 1999. The outcome data were event reports obtained by sending questionnaires (green forms) to the prescribing doctor at least 6 months after the first prescription for an individual patient. Incidence densities, expressed as number of first reports of an event/1000 patient-months of exposure, were calculated. Significant differences between incidence densities (IDs) for events reported in the 1st month (ID1) and months 2 and 3 (ID2–3) of exposure were regarded as potential signals. Reasons for stopping orlistat were analysed. Follow-up information was requested for selected events and used to assess the causal association with orlistat.Results:Green forms containing clinically useful information on 16 021 patients (median age 45 years (interquartile range 35–54); 80.1% females) were received. The events reported most frequently during the 1st month of treatment were ‘not effective’ (639; 4.0% of cohort), diarrhoea (371; 2.3%) and weight loss (230; 1.4%). Twelve clinical adverse events were identified for which ID1 was significantly greater than ID2–3. These included non-specific events (e.g. intolerance, malaise/lassitude, unspecified side effects), weight loss and vaginitis/vulvitis. The remaining events were gastrointestinal in nature and included diarrhoea, pain abdomen, flatulence, nausea/vomiting, rectal discharge, faecal incontinence and ‘gastrointestinal unspecified’ events. A similar pattern of predominately gastrointestinal events was seen for reasons for stopping and suspected adverse drug reactions. Review of selected events for causality revealed 45 events which were assessed as possibly or probably related to orlistat.Conclusions:This study shows that orlistat is fairly well tolerated. The safety profile of orlistat was similar to the prescribing information and experience reported in the literature.International Journal of Obesity (2006) 30, 1645–1652. doi:10.1038/sj.ijo.0803323; published online 21 March 2006 [ABSTRACT FROM AUTHOR]