The study had several strengths. Firstly, we are likely to have identified all potential diagnosed cases of non-fatal venous thromboembolism in the study cohort, because information about outpatient and emergency department visits and hospital admissions in the UK is sent to general practitioners, and the practices contributing to the General Practice Research Database are trained to record the diagnoses associated with such visits and admissions in the patient’s record. Secondly, referral and diagnostic biases seem unlikely explanations for our results, as an elevated risk in users of the drospirenone oral contraceptive was also apparent in the analysis confined to cases of pulmonary embolism—cases in whom the severity of the condition makes any potential differential referral of users of the newer oral contraceptive much less likely. Thirdly, for both cases and controls, information about the medical history and prescription of oral contraceptives and other drugs came from medical records and hence was not subject to recall bias. Body mass index also came from records, rather than self report. Controls came from the same general practices as cases and were matched on the number of years of recorded medical information, ensuring comparable quality and duration of information. Moreover, the duration of recorded information was considerable (mean duration 10 years), permitting the ascertainment of comprehensive medical and contraceptive histories.