Oral contraceptive switching patterns in the United Kingdom: an important potential confounding variable in studies of venous thromboembolism

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Abstract
Objective: To investigate whether patients changing from their current oral contraceptive (OC) are preferentially switched to third-generation preparations.Method: The database comprised 12 months of medical records generated during 1990-91 from 693705 patients and 153687 OC users aged 14-45 years registered with 398 practices in the United Kingdom. Subjects who switched OC preparations were identified.Results: A total of 16197 women (10.5%) switched OC preparations at least once during the study period. There was a statistically significant net switching from second-generation OCs to third generation OCs with a ratio of 1.19 (95% C l, 1.12-1.26). This switching behavior took place in both younger(< 25 years) and older(> 25 years) OC users. In addition, a significant net switching was observed from progestogen-only pills to both second- and third-generation OCs. This was more pronounced in women >25 years.Conclusion: Bias resulting from preferential switching of women from second-generation to third-gener...