Results of a large, retrospective study support the association between newer contraceptive pills and a higher risk of venous thromboembolism (VTE).
The research showed that pills containing one of the newer types of progestogen—drospirenone, desogestrel, gestodene, and cyproterone—are associated with a nearly 2-fold higher risk of VTE than pills containing older progestogens—levonorgestrel, norethisterone, and norgestimate.
The researchers said this study has sufficient power to provide reliable comparative findings for different formulations of combined oral contraceptives. However, because it is an observational study, no definitive conclusions can be drawn about cause and effect.
The team described this research in BMJ alongside a related editorial.
Although the increased risk of VTE associated with combined oral contraceptives has been suggested previously, prior studies have used different methods to examine this link. So the relative risks associated with different combinations remain inconclusive.
Yana Vinogradova, of the University of Nottingham in the UK, and her colleagues tried to address these differences to help explain the range of results.
The team used prescription data from 2 large UK general practice databases to measure the associations between the use of combined oral contraceptives and the risk of VTE in women aged 15 to 49, adjusting for other known VTE risk factors.
The researchers matched 10,562 women with VTE to 42,034 control subjects and found that women who used any combined oral contraceptive within the past year had an increased risk of VTE compared with non-users of similar age and health status. The adjusted odds ratio was 2.97.
The risk of VTE was significantly higher for women who used the newer oral contraceptives than the older pills (P<0.001). The adjusted odds ratios were 4.28 for desogestrel, 4.27 for cyproterone, 4.12 for drospirenone, and 3.64 for gestodene, compared to 2.38 for levonorgestrel, 2.53 for norgestimate, and 2.56 for norethisterone.
The number of extra VTE cases per year per 10,000 treated women was lowest for levonorgestrel and norgestimate (6 cases for both) and highest for desogestrel and cyproterone (14 cases for both).
The researchers said that, although this is an observational study, it has produced the most reliable possible VTE risk estimates using currently available UK prescription data.
