An increased risk of venous thromboembolic and thrombotic disease associated with the use of oral contraceptives is well established. The risk of venous thrombotic and thromboembolic events is further increased in women with conditions predisposing for venous thrombosis and thromboembolism. Case control studies have found the relative risk of users compared to non-users to be 3 for the first episode of superficial venous thrombosis, 4 to 11 for deep-vein thrombosis or pulmonary embolism, and 1.5 to 6 for women with predisposing conditions for venous thromboembolic disease. Cohort studies have shown the relative risk to be somewhat lower, about 3 for new cases and about 4.5 for new cases requiring hospitalization. The approximate incidence of deep-vein thrombosis and pulmonary embolism in users of low dose (<0.05 mg ethinyl estradiol) combination oral contraceptives is up to 4 per 10,000 woman-years compared to 0.5 to 3 per 10,000 woman-years for non-users. However, the incidence is less than that associated with pregnancy (6 per 10,000 woman-years). The excess risk is highest during the first year a woman ever uses a combined oral contraceptive. Venous thromboembolism may be fatal. The risk of thromboembolic disease due to oral contraceptives is not related to length of use and gradually disappears after pill use is stopped. 

A post-marketing observational study evaluated the risk of venous thromboembolism with Amethyst™ use in two large US automated healthcare claims databases. The study was not completed as planned due to low accrual of Amethyst™ users in these databases and discontinuation of the product from the market due to low usage. At study discontinuation, the crude incidence rate of venous thromboembolism among Amethyst™ users (n=12,281) was 17.6 per 10,000 person-years, compared to 8.8 per 10,000 person-years among the users of cyclic oral contraceptives containing 20 mcg of ethinyl estradiol and a progestogen, and 5.1 per 10,000 person-years among the users of cyclic oral contraceptives containing the progestin levonorgestrel and 20 mcg of ethinyl estradiol. Adjustment for important risk factors or confounders (such as obesity, cardiovascular disease and other diseases) for venous thromboembolism could not be performed due to the small sample size. Although the study results suggest an elevated risk of venous thromboembolism with current Amethyst™ use compared to cyclic oral hormonal contraceptive use, reliable interpretation of the results is significantly limited due to the small sample size and concerns over unmeasured and uncontrolled confounding, as well as questions about the suitability of the comparator selection and the validity of the venous thromboembolism definition.

A two-to-four fold increase in relative risk of postoperative thromboembolic complications has been reported with the use of oral contraceptives. The relative risk of venous thrombosis in women who have predisposing conditions is twice that of women without such medical conditions. If feasible, oral contraceptives should be discontinued at least four weeks prior to and for two weeks after elective surgery of a type associated with an increase in risk of thromboembolism and during and following prolonged immobilization. Since the immediate post-partum period is also associated with an increased risk of thromboembolism, oral contraceptives should be started no earlier than four weeks after delivery in women who elect not to breastfeed, or after a midtrimester pregnancy termination.