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These alternate hormone-releasing birth control methods include skin patches, implants and vaginal rings. To reduce the risk, women who use these should consider switching to the pill, the researchers said.
Deep vein thrombosis is a kind of clot that typically originates in the legs and can travel to the lungs, where it becomes an often deadly pulmonary embolism. Both types of clots combined are called venous thrombosis, according to the study. Symptoms include leg pain, chest pain or sudden shortness of breath.
"The transdermal patch and vaginal ring confer at least a sixfold increased risk of venous thrombosis as combined pills with desogestrel or drospirenone, a risk which is about twice the risk among women using second-generation pills with levonorgestrel," said lead researcher Dr. Ojvind Lidegaard, a professor of obstetrics and gynecology at the University of Copenhagen.
However, hormone-releasing intrauterine devices (IUDs) do not increase the risk of venous thrombosis, he said.
"Women should be informed about these risks in order to be able to choose the most appropriate hormonal contraceptive product," Lidegaard said. "There are hormonal contraceptive alternatives which confer less or no risk of venous thrombosis."
Methods like the sponge (Today Sponge), which uses a spermicide rather than hormones, were not addressed in the study.
The findings were published May 10 in the online edition of the BMJ.
For the study, Lidegaard's team collected data on incidents of venous thrombosis in Danish women using methods of birth control other than the pill. All the women were aged 15 to 49, and none were pregnant.
The researchers found that between 2000 and 2010 there were more than 3,400 diagnoses of venous thrombosis.
For women who did not use any type of hormonal contraceptive, two women developed clots for every 10,000 (combined) years they used contraceptives.
For women taking the pill containing the hormone levonorgestrel, the risk for a clot was three times higher, or 6.2 clots for every 10,000 years they took the pill, the researchers found.
The risk to women who used a skin patch was about eight times higher, or 9.7 clots per 10,000 exposure years.
Women who used a vaginal ring had a 6.5 times higher risk, or 7.8 events per 10,000 exposure years).
For women who used an implant that contained only progestogen, the increased risk for clots was very small. There was no risk for women using a progesterone-only IUD and it may have has a protective effect, the researchers noted.
There was no reduction in risk with the long-term use of a patch or vaginal ring, they added.
"For the majority of young women, the recommendation is second-generation combined pill with levonorgestrel, and for women who have given birth, that a hormone-releasing intrauterine device is an attractive option, because it at the same time does not increase the risk of venous thrombosis, perhaps even protects against them, and reduces menstrual complaints," Lidegaard said.
To cut the number of women who develop clots from these riskier birth control methods, the authors advised that more women choose the pill.
Dr. Elizabeth Poynor, a gynecologist and pelvic surgeon at Lenox Hill Hospital in New York City, commented that "over the past years, newer versions of combined hormonal contraception have been developed."
"While these types of transdermal hormonal contraceptives may be more convenient for some women and lead to better compliance with appropriate use, they have their own set of medical side effects and concerns," she said.
"This highlights the importance of reviewing the risks, benefits and alternatives among differing types of medications within the same broad category," Poynor added.
For some women, these types of medications are extremely useful, but their potentially elevated risk of thrombosis should be reviewed before prescribing them, Poynor stressed.
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