Most users of hormonal birth control have heard warnings that the drugs may increase risk of blood clots. Two new studies in the journal BMJ examine this risk, and attempt to characterize the amount of risk by how long women were on the drugs and the type of hormone. In general, they find the highest risk when women first start taking the pill, decreased risk with decreased dose, and a lower risk with levonorgestrel than with other progestagens.
The first, a case-control study conducted in the Netherlands, compared 1,524 pre-menopausal women who had experienced a deep vein thrombosis or pulmonary embolism with 712 women who had not (the control group). All of the participants completed a questionnaire on risk factors for venous thrombosis such as family history, pregnancy, and oral contraceptive use, and were further interviewed about their oral contraceptive use.
The researchers report a fivefold increase in risk for venous thrombosis associated with oral contraceptive use. The also found different levels of risk by the type of contraceptive in use – compared to women who did not use the drug, those using a contraceptive containing levonorgestrel (the most frequently used in the study) had an almost fourfold increase in risk (odds ratio 3.6). Risk also appeared to increase 5.6-fold for those containing gestodene, 6.3-fold for drospirenone, 6.8-fold for cyproterone acetate, and 7.3-fold for desogestrel. Risk seemed to increase with higher dosages, and to decrease over time – women had the greatest risk as new users in the first 3 months (odds ratio 12.6) which declined to the average 5-fold increased risk after one year.
The second is a cohort study using registry data to track Danish women over time for various types of thromboses and oral contraceptive use. The paper uses “woman years” as a measure of the participants and explains that the study covered 10.4 million woman years of observation (2 years of observation of 5 people, for example, would be 10 person-years). They report that “The overall absolute risk of venous thrombosis per 10,000 woman years in non-users of oral contraceptives was 3.01 and in current users was 6.29.”
These authors also found decreased risk over time, with the highest risk in the first year, a decreased risk with decreased estrogen dose, and a higher risk with contraceptives containing desogestrel, gestodene, or drospirenone were associated with a higher risk of venous thromboembolism than those containing levonorgestrel.
The BMJ issue also includes an editorial on the studies – which notes that while the absolute risk of blood clots is low, providers may need to consider these findings in selecting contraceptives with individual patients – and links to a recent review of different contraception options for women.