BMJ 1996;312:577 (2 March)

Letters

Postmarketing surveillance does not catch all adverse events

EDITOR,--H B M Reijnen and W J Atsma, of Organon Pharmaceuticals, use data from the company's product surveillance database to show that the risk of venous thromboembolism with the pills Marvelon and Mercilon, both of which contain desogestrel, is highest in the first few months of use and falls dramatically thereafter.1 The World Health Organisation's recent multicentre case-control study refutes this suggestion: the duration of current use of oral contraceptives did not alter the risk.2

What is worrying about the authors' letter is the fact that the product surveillance database contains data on only 434 adverse drug events, fewer than 100 of which are venous thromboemboli. With over 36 million woman years of exposure to these two pills, we can conservatively estimate that the total number of venous thromboemboli is over 5000. Thus the manufacturer's database has information on less than 2% of all such cases. The impressive graph accompanying . . . [Full text of this article]


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