Women who use a vaginal ring or skin patch as contraceptive methods are at around double the risk of a blood clot compared to those who take the second generation of contraceptive pills, according to a large-scale study.
Research published on bmj.com has shown women using the skin patch have an eight times increased risk of venous thrombosis and those using the vaginal ring a 6.5 times increased risk compared with women who do not use any type of hormonal contraception.
This compares with a threefold increase risk of a blood clot among women taking a combined oral contraceptive pill containing the hormone levonorgestrel.
The study found that using the progestogen-only subcutaneous implant carried a slightly increased risk, while use of a progestogen-only intrauterine device did not confer any risk and might even have a protective effect.
A team led by Professor Xjvind Lidegaard at the University of Copenhagen reviewed data on non-oral hormonal contraceptive use and first-ever venous thrombosis in all Danish non-pregnant women aged 15-49 from 2001 to 2010.
The results were based on 9,429,128 observation years during which 3,434 confirmed diagnoses of first-ever venous thrombosis were recorded.
The risk of venous thrombosis among women who did not use any type of hormonal contraception and who were aged 15-49 was on average two events per 10,000 exposure years, according to the study.
Women taking a combined oral contraceptive pill containing the hormone levonorgestrel had a three times increased risk of 6.2 events per 10,000 exposure years.
Compared with non-users of the same age, women who used a skin patch had an eight times increased risk, 9.7 events per 10,000 exposure years, while women who used a vaginal ring had a 6.5 times increased risk, at 7.8 events per 10,000 exposure years.
Unlike combined pills, no reduction in risk was seen with long-term use of a patch or vaginal ring.
Commenting on the findings, Professor Lidegaard said women aged from 30 upwards who use the vaginal ring or the patch might consider shifting to using the second generation of contraceptive pills.
He said: "The most important thing is that women are informed about the risk. The ring and the patch is approximately double the risk compared with second generation pills.
"If women still prefer to have a ring or a patch, for example because they are not able to remember to take the pill daily, then they can continue. That is their own choice.
"For me, the important thing is that they are informed about the risk."