Women who take newer birth control pills appear to get dangerous blood clots in their veins about twice as often as women on an older pill formulation. That was the finding of one of the largest studies ever to look at the link between blood clots and hormonal contraception.
Still, the newer pills carry a low risk of venous thromboembolism (VTE), experts stressed, and women should consult with their doctors before changing their birth control.
Blood clots are a well-known side effect of birth control pills.
When a clot forms in a vein, often in a leg, part of the clot can break free and travel to other parts of the body. A blood clot that travels to the lungs or brain, for example, can be fatal.
For the study, researchers linked 15 years of medical, hospital, death, and prescription drug records for all women in Denmark between the ages of 15 and 49.
Records on nearly 1.3 million women were included in the study. About 30% of them had never used hormonal contraception, while nearly 70% had used some form of hormonal birth control.
There were 4,307 cases of blood clots that required treatment reported in the study. Most (64%) were deep vein thrombosis (DVT), where a blood clot chokes circulation in a limb, causing a leg or arm to become swollen, stiff, or painful. About one-quarter were blood clots in the lungs. About 2% had blood clots that caused strokes.
For women who had never used any hormonal birth control, about 3.7 out of 10,000 were diagnosed with a blood clot in a vein in a year's time.
Being on an older-generation pill that contained an estrogen and the progestin hormone levonorgestrel roughly doubled that risk, to 7.5 women out of 10,000 followed for one year.
Being on the newest kinds of pills, which contain the progestin hormones drospirenone, desogestrel, or gestodene along with estrogen, doubled the risk again, making it six to seven times as high as women who weren't using hormonal forms of birth control.
Still, on average, about 10 out of 10,000 women taking newer kinds of birth control pills had venous thromboembolism in a year's time.
Although that's a serious increase, it is still only half as high as the risk of blood clots seen in women who are pregnant or who have recently had a baby.
Drospirenone is in the contraceptive pills Beyaz, Gianvi, Loryna, Ocella, Safyral, Syeda, Yasmin, Yaz, and Zarah.
Desogestrel is in Ortho-Cept, Cyclessa, Deogen, Emoquette, Kariva, Mircette, and Velivet.
Gestodene is not approved for use in the U.S.
The study, which is published in BMJ, comes as the FDA is reviewing the safety of newer birth control pills.
That investigation, announced in May, was scheduled to be finished by the end of summer.
In September, the agency said it had not yet reached a conclusion but "remains concerned about the potential increased risk of blood clots with the use of drospirenone-containing birth control pills."
A panel of experts is scheduled to meet in December to discuss the findings of an FDA-funded study that evaluated the risks of blood clots in women who used several different hormonal birth control products.
Bayer, the maker of Yaz, Yasmin, Ocella, and Beyaz, presented this statement to WebMD regarding the research:
"This study represents a reanalysis of the retrospective cohort study of Lidegaard et al., initially published in 2009 investigating the risk of venous thromboembolism (VTE) of combined oral contraceptives (COCs). Bayer is currently evaluating this publication and cannot comment at this time.
"Clinical data from a period of more than 15 years and up to 10 years of post-marketing safety study results support Bayer's assessment that its drospirenone-containing COCs are safe and effective when used as indicated and that the risk of VTE is similar to any other low-dose estrogen COC studied, regardless of the progestogen."
Advice to Women
"This is one of several studies that have shown that certain birth control pills have higher risks of blood clots over other birth control pills," says Jennifer Wu, MD, an obstetrician/gynecologist at Lenox Hill Hospital in New York City, who was not involved in the research.
"I think women really need to talk with their doctors before they start a birth control pill, and doctors should try to choose ones that have lower risks," Wu says. "I wouldn't start with these riskier oral contraceptives as first-line, first-start pills."
Because so many women take birth control pills, even small risks can have significant public health consequences.
"You have to consider that 200 million women, every day, worldwide take such a pill. So even if it's only one in 500 per year who get the thrombosis if they are on a fourth-generation pill and are 30 years old, then you actually get a relatively high number of complications," says researcher Ojvind Lidegaard, MD, professor of obstetrics and gynecology, Rigshospitalet at the University of Copenhagen, in Denmark. "And you could actually halve that number just by changing the pill from a fourth- to a second-generation pill."
But experts say switching to an older pill may not be the best option for every woman.
"It is important to have a range of different oral contraceptives available because some women tolerate one preparation better than another," Philip C. Hannaford, MD, who is the Grampian Health Board chair of primary care at the University of Aberdeen, in Scotland, says in an email to WebMD.
Some women prefer one kind of pill over another, Hannaford says, because they experience less nausea, acne, or weight gain.
"This means that clinicians and women often chose to use combined oral contraceptives which do not contain levonorgestrel, and this seems a pragmatic and sensible thing to do given that the background risk of DVT is very low," says Hannaford, who was not involved in the research but wrote an editorial on the study.
Smoking, being overweight, family history, and age also increase the risk of blood clots.
Those risks should be discussed with a doctor before starting or switching any kind of hormonal contraception.
Women who are on birth control pills should also be aware of the symptoms of blood clots. Symptoms of a blood clot in the leg can include pain in the calves from walking and one leg swelling larger than the other.
"One of the symptoms patients really need to look out for is shortness of breath because one of the huge risks with DVT is that the clot can travel to the lungs, and that can kill people," Wu says. "Even though the risk overall is low, I think that the possible consequences are pretty dire. Patients on these riskier birth control pills should know the symptoms to look for."