THURSDAY, Oct. 27 (HealthDay News) -- The U.S. Food and Drug Administration on Thursday said it "remains concerned" that a newer generation of birth control pills may raise the odds for serious blood clots more than older forms of the Pill.

The announcement concerns oral contraceptives containing a newer type of progestin hormone called drospirenone, which includes Bayer's Yaz or Yasmin. According to the FDA, the new study found a higher risk of venous thromboembolisms (VTEs) -- potentially dangerous clots -- in women on the drospirenone-containing pills vs. those on older forms of oral contraceptives.

The FDA-funded review involved the medical histories of more than 800,000 American women, all of whom were on some type of birth control between 2001 and 2008. The study found that women taking the newer oral contraceptives experienced a higher rate of clots than women on older forms of the contraceptive pill.

The review also found that women on two other forms of birth control -- the Ortho Evra patch from Johnson & Johnson and the NuvaRing vaginal ring from Merck -- had a higher rate of clots.

For now, the FDA is not advising that most women switch to another form of contraception. "If your birth control pill contains drospirenone, do not stop taking it without first talking to your health care professional," the agency said. "Contact your health care professional immediately if you develop any symptoms of blood clots, including persistent leg pain, severe chest pain or sudden shortness of breath. If you smoke and are over 35 years of age, you should not take combination oral contraceptives because they increase the risk that you could experience serious cardiovascular events, including blood clots."

Thursday's announcement was not the FDA's first word on this issue, nor is likely to be the last. The agency issued a similar warning in September, and in a statement released Thursday said that, "given the conflicting nature of the findings from six published studies evaluating this risk, as well as the preliminary data from the FDA-funded study," it plans to host a public meeting on the issue on Dec. 8.

The announcement Thursday comes a day after the release of a study in BMJ that also found newer birth control pills were tied to a higher risk for clots.

In that study, researchers reviewed data on all Danish women, aged 15 to 49, who were not pregnant between January 2001 and December 2009. During that time, more than 4,200 first episodes of VTEs occurred.

Women taking birth control pills with a newer progestin hormone had twice the risk of clots compared to those who took the older form of contraceptive pills.

Compared to women who did not use birth control pills, the risk of VTE was three times higher among those who used pills with levonorgestrel and six times higher among those who took pills with drospirenone, desogestrel or gestodene.

But the absolute risk of VTE associated with taking the newer pills remained relatively low, about 10 per 10,000 women, according to the University of Copenhagen researchers.

For every 2,000 women who switched from using newer pills to pills with levonorgestrel, there would be one less case of clots a year.

While some doctors may choose to prescribe birth control pills with a lower risk whenever possible, it is crucial not to exaggerate the risk of VTE, Dr. Philip Hannaford of the University of Aberdeen in Scotland, wrote in an accompanying editorial in the journal.

"Oral contraceptives are remarkably safe and may confer important long-term benefits in relation to cancer and mortality," he said in a journal news release.

Dr. Glenn Jacobowitz, vice chair of the division of vascular surgery at NYU Langone Medical Center in New York City, said: "The information for Yaz is not new. That has recently already been shown in studies to have an increased risk of blood clots than other oral contraceptives. The information on NuvaRing and Ortho Evra would be a new, but similar finding. This is certainly worrisome, particularly for women over age 35 and for smokers."

More information

The American College of Obstetricians and Gynecologists has more about birth control pills.