The British Medical Journal has published the results of a cohort study that aimed to assess the risk of venous thrombosis in current users of non-oral hormonal contraception.
The study reviewed data from four national registries in Denmark containing information for 1,626,158 women aged 15 to 49. Individuals with a history of cancer or previous venous thrombosis were excluded.
The study included 9,429,128 women years of data between 2001 and 2010 with 5,287 first venous thrombosis events recorded of which 3,434 were confirmed. The incidence of venous thrombosis in non-users of hormonal contraception was 2.05 per 10,000 women years.
Compared to this baseline risk the relative risk of venous thrombosis for contraceptives was as follows:
- Combined oral contraceptives containing levonorgestrel - 3.21 (95% CI 2.70 - 3.81)
- Combined oral contraceptives containing norgestimate - 3.57 (95% CI 2.98 - 4.21)
- Contraceptive Patch - 7.90 (95% CI 3.54 - 17.65)
- Vaginal Ring - 6.48 (95% CI 4.69 - 8.94)
- Implant - 1.40 (95% CI 0.58 - 3.38) non-significant
- Levonorgestrel IUD - 0.57 (95% CI 0.41 - 0.81)
The increased relative risk observed with the patch is equivalent to 7.66 additional events of venous thrombosis per 10,000 women years.
The authors note that the data sources used are robust and allow of limitation or elimination of several potential sources of bias. However, they also note that the data did not allow for any correction for family history of venous thrombosis or body mass index.
Action: Clinicians should be aware of this study. The data in this study may be useful in communicating risk to aid individual patients in making contraceptive choices.
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