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

Abstract

Bias resulting from preferential switching of women from second-generation to third-generation OCs may have influenced the results of recently published studies on combined OCs and the risk of venous thromboembolism. Studies of venous thromboembolism in women using OCs should either restrict from the study women who have switched preparations or should include switching as a variable in any analysis.