Newer forms of birth control pills may carry a higher risk of serious blood clots than earlier oral contraceptives.
Women taking the "fourth generation" pills containing drospirenone, a new type of progestogen hormone, had double to triple the risk of blood clots compared to women taking levonorgestrel-containing pills, according to two studies published online April 22 in BMJ.
"This is confirming what a lot of physicians had suspected for some time. The new pills do have a higher clot risk. But it's still much lower than the risk associated with pregnancy, so it doesn't preclude us using it," said Dr. Rachel Bonnema, assistant professor of internal medicine at the University of Nebraska Medical Center in Omaha.
Added Dr. Steven R. Goldstein, professor of obstetrics and gynecology at NYU Langone Medical Center in New York City: "Even if the findings turn out to be real, we're talking about an increase from a very small risk to a very small risk."
These new pills -- marketed as Yaz or Yasmin in North America -- are popular, although the risk of blood clots, also known as venous thromboembolism, has been noted before.
One of the two new studies involved U.S. women aged 15 to 44 who took a contraceptive pill containing either drospirenone or levonorgestrel after January 2002.
In that study, the researchers, led by Dr. Susan Jick of Boston University School of Medicine, compared 186 women who had a blood clot with 681 who had not.
Those taking the newer pill had a 2.3 times greater risk for a blood clot. The absolute risk, however, was still small -- 30.8 per 100,000 among those taking drospirenone, compared to 12.5 per 100,000 in women taking levonorgestrel.
The other study looked at similarly aged women in the United Kingdom and found a three-fold elevated risk for blood clots among women taking the newer version of the pill. That translated to 23 per 100,000 women in the drospirenone group and 9.1 per 100,000 women in the levonorgestrel group.
"The absolute risk per 100,000 women is low," said Dr. Jill Rabin, chief of ambulatory care, obstetrics and gynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y.
It wasn't clear why drospirenone might increase risk, she added.
Another recent study concluded that the fourth-generation pill carried no increased risk of gallbladder disease, which also had been of some concern.
Will the current findings change medical practice? Maybe, but probably not much.
"If I have a patient coming in tomorrow starting on birth control, I might not reach for the Yaz or Yasmin product," said Goldstein. "But I definitely, definitely would not take anybody off Yaz or Yasmin who's been on it six months or a year and is doing well, who has no family history or personal history of venous thromboembolism."
People with an increased risk of blood clots probably shouldn't be on any birth control pill, he added.
The drospirenone version does have advantages, he said.
"The water retention tends to be dramatically less, and the low dose form is approved for PMDD [premenstrual dysphoric disorder]," Goldstein said. "There are definitely advantages for the right people. I would not throw these pills out based on this."
Bonnema said the new findings would be helpful in counseling patients, but added, "I don't know if it's of utmost concern."