ORAL-CONTRACEPTIVE ESTROGEN AND PROGESTIN POTENCIES AND THE INCIDENCE OF DEEP VENOUS THROMBOEMBOLISM

To assess possible differences in the incidence of venous thrombosis and pulmonary embolism associated with oral contraceptives of varying hormonal potencies, the authors conducted a retrospective cohort study in the 15-44 year old Michigan Medicaid population. Cohorts were defined by the progestin- and oestrogen-potencies of oral contraceptives in use at the time of follow-up as classified by an oral contraceptive potency scheme. Using the low-oestrogen-/low-progestin-potency formulations for reference (rate ratio = 1), adjusted rate ratios of 0.8 (95% Cl: 0.5 to 1.3, P = 0.41) and 0.6 (95% Cl 0.4 to 1.2, P = 0.13) were observed for intermediate-progestin-potency and high-progestin-potency formulations, respectively. Adjusted rate ratios of 1.4 (95% Cl: 0.8 to 2.3, P = 0.21) and 2.6 (95% Cl: 1.2 to 5.5, P = 0.01) were observed for intermediate- and high-oestrogen-potency formulations. These data suggest a dose-response relationship between oral contraceptive oestrogen potency and venous thromboembolism, whereas no such evidence for a dose-response relationship between oral contraceptive progestin potency and venous thromboembolism was found.