Intended for healthcare professionals


Pitfalls of pharmacoepidemiology

BMJ 2000; 321 doi: (Published 11 November 2000) Cite this as: BMJ 2000;321:1171

Oral contraceptive studies show a need for caution with databases

  1. David C G Skegg, professor
  1. Department of Preventive and Social Medicine, University of Otago, PO Box 913, Dunedin, New Zealand

    General practice p 1190

    Three months ago a paper in the BMJ analysed the incidence of venous thromboembolism before and after the warning from the UK Committee on Safety of Medicines about third generation oral contraceptives.1 Using computer records of general practitioners, Farmer et al found that the incidence among pill users had not dropped, and they concluded that their findings were not compatible with a doubling of risk in women using third generation contraceptives (compared with older preparations). Their paper received wide publicity because it called into question an emerging consensus about this issue.2

    This week's BMJ contains another analysis of computer records from British general practice, conducted by a group in Boston (p 1190).3 Jick et al found that, both before and after the warning in October 1995, the risk of venous thromboembolism in women using third generation oral contraceptives was about twice that in users of preparations containing levonorgestrel. Moreover, fewer cases occurred after the warning than would have been expected if the prescribing of oral contraceptives had not changed.

    What is remarkable is that these two studies, reporting opposite conclusions, both used the same General Practice Research Database.4 …

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