The risk of developing venous thromboembolism is generally greater for women using oral contraceptives with newer types of progestogen hormones than for those taking older, second-generation birth control pills, study results showed.
“Women exposed to drospirenone, gestodene, cyproterone, and desogestrel within the last 28 days had around a four times increased risk of venous thromboembolism,” the investigators found. Women exposed to levonorgestrel, norethisterone, and norgestimate had about a 2.5 times greater risk of venous thromboembolism than did women not exposed in the past year, said Yana Vinogradova and her colleagues at the University of Nottingham (England) (BMJ. 2015 May 26 [doi:10.1136/bmj.h2135]).
The researchers conducted two nested case-control studies using data from 618 primary care practices in the Clinical Practice Research Datalink (CPRD) and 722 practices in the QResearch primary care database. A total of 5,062 cases from CPRD and 5,500 cases from QResearch were matched one to five with 19,638 and 22,396 controls, respectively.
Approximately 29% of CPRD patients and 26% of QResearch patients used oral contraceptives, most commonly levonorgestrel. Overall, any use of combined oral contraceptives resulted in a three times increased risk for venous thromboembolism, compared with no use in the past year.
After accounting for smoking, obesity, a wide range of other health conditions, alcohol consumption, polycystic ovary syndrome and recent infections, surgeries, leg/hip fractures, and hospital admission, the researchers calculated an increased odds ratio for each hormone: desogestrel (4.28), cyproterone (4.27), drospirenone (4.12), gestodene (3.64), levonorgestrel (2.38), norgestimate (2.53), and norethisterone (2.56). The increased VTE risk in patients on these hormones was compared with no exposure to oral contraceptives in the previous year.
In terms of numbers needed to harm, the researchers estimated that use of levonorgestrel and norgestimate resulted in 6 extra cases of VTE each year per 10,000 treated women aged 15-49, and 7 extra cases for women aged 25-49.
Desogestrel and cyproterone each contributed 14 additional cases of VTE each year per 10,000 treated women aged 15-49, and drospirenone, desogestrel, and cyproterone each contributed to an extra 17 cases of VTE each year per 10,000 women aged 25-49.
“We believe this study has the statistical power and sufficient adjustment for relevant confounders to be regarded as an important clarifying study, which has produced the most reliable possible risk estimates using currently available U.K. prescription data,” the researchers wrote.
There was no external funding for the study. Julia Hippisley-Cox is the unpaid director of QResearch, a not-for-profit organization that is a joint partnership between the University of Nottingham and EMIS, a commercial IT supplier. She is also a paid director of ClinRisk, which produces clinical risk algorithm-related software.