Two new studies add to the growing evidence that birth control pills containing a newer type of progestin may put some women at higher risk for blood clots.
Both studies, published today in the British Medical Journal, found that women who took pills containing drospirenone -- which includes the brands Yasmin, Yaz and Ocella -- more than doubled their risk of nonfatal blood clots when compared with those who took pills containing levonorgestrel, an older form of progestin, included in the brands Levlite or Levlen.
But experts advise caution in interpreting these results: The overall risk for blood clots with any birth control pill -- whether it contains drospirenone or levonorgestrel -- remains low, said Dr. Lauren Streicher, an assistant professor of obstetrics and gynecology at the Feinberg School of Medicine at Northwestern University.
"While there may be an increase in blood clots, it's important to keep in mind that the likelihood of developing a blood clot is so low in a low-risk population that a two- to three-fold increase may not translate to large absolute numbers," said Streicher.
Pregnancy actually puts women at higher risk for blood clots than birth control pills -- a blood clot occurs in one pregnant woman for every 1,000 to 1,500 pregnant women, while one in 3,000 women who take birth control pills experience some form of blood clot, according to the National Blood Clot Alliance.
Oral contraceptives contain a combination of estrogen and progestin, with progestin being the strongest agent in the mix. Estrogen is added to offset some of the side effects of progestin. While previous studies found that estrogen increased some women's risk for blood clots, other studies suggested that, to a lesser degree, progestin could too.
But not all doctors are convinced.
Dr. David Grimes, vice president of Family Health International, says he's not persuaded that either form of progestin -- drospirenone or levonorgestrel -- has been found to cause blood clots.
"These data are not credible. There's no biological basis on progestin causing blood clots," said Grimes. Grimes served as a member of the data safety monitoring committee at Bayer, which manufactures Yaz and Yasmin.
"They're barking up the wrong tree," he said.
While the studies published today make it difficult to determine the blood clot risk, doctors need to continue to inform their patients about any potential risk from any form of estrogen and progestin combination birth control pills, said Dr. Ricardo Azziz, a reproductive endocrinologist and researcher at Georgia Health Sciences University. And women who are obese, older than 35 and smoke or who have a family history of blood clots should not take oral contraceptives at all.
"It may be best to consider the data when choosing the first pill that a woman starts," said Azziz. Women respond to birth control pills differently, and side effects play into the decision about which pill to take. Some women experience weight gain, mood swings or spotting while on the pill.
Birth control pills with drospirenone -- the newer progestin -- have been prescribed more frequently because many women tolerate it better than the older progestin, said Streicher. The U.S. Food and Drug Administration has approved drospirenone to treat premenstrual dysphoric disorder, a condition marked by severe depression, irritability and tension.
According to Azziz, women are more likely to continue taking the pill regularly if they find one that provides the least number of side effects -- and the largest number of secondary beneftis.