Intended for healthcare professionals

Letters The pill and thrombosis

Study subject to unmeasured confounders and biases

BMJ 2011; 342 doi: (Published 31 May 2011) Cite this as: BMJ 2011;342:d3349
  1. Anne Szarewski, clinical senior lecturer1,
  2. Diana Mansour, clinical director2
  1. 1Wolfson Institute of Preventive Medicine, Queen Mary, University of London, London EC1M 6BQ, UK
  2. 2Newcastle Hospitals Community Health, New Croft Centre, Newcastle upon Tyne NE1 6ND, UK
  1. a.szarewski{at}

The papers on venous thromboembolism in oral contraceptive users have similar faults to earlier database studies.1 2 These include lack of validation of venous thromboembolism (VTE) cases and lack of information on, or control of, important potential confounders such as duration of use, especially short duration use; family history; body mass index (BMI); and smoking.3 Jick and Hernandez’s …

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