Revised labels on some types of birth control to advise of a possibly higher risk of blood clots have been ordered by US authorities. This includes German pharmaceutical giant Bayer's Yaz pills.
"Women who use birth control pills with drospirenone (like Yaz) may have a higher risk of getting a blood clot," said a new Yaz label on the US Food and Drug Administration website.
"Some studies reported that the risk of blood clots was higher for women who use birth control pills that contain drospirenone than for women who use birth control pills that do not contain drospirenone."
However, the regulatory agency pointed out that studies have shown mixed results.
"The studies reviewed did not provide consistent estimates of the comparative risk of blood clots between birth control pills that contain drospirenone and those that do not," the FDA said in a statement.
"The studies also did not account for important patient characteristics (known and unknown) that may influence prescribing and that likely affect the risk of blood clots," it added.
"For these reasons, it is unclear whether the increased risk seen for blood clots in some of the epidemiologic studies is actually due to drospirenone-containing birth control pills."
Drospirenone is a synthetic progestin which can suppress ovulation and thereby prevent pregnancy either when used alone or in combination with an estrogen component.
There are 11 approved versions of such pills on the US market, including names such as Gianvi, Loryna, Ocella, Safyral, Syeda, and Zarah.
In December, a panel of experts that advises the FDA urged stronger labeling on drospirenone-containing pills, voting 21-5 that the current labels did not adequately reflect the risks and benefits.
The panel was divided on the question of whether the benefits of such pills outweighed the risks, with 15 saying "yes" and 11 saying "no."
The pills have been marketed on the basis they help alleviate premenstrual symptoms, reduce acne and cause less weight gain than competitors. They have also been the focus of numerous lawsuits.
Two recent studies in the British Medical Journal found that drugs like Yaz and Yasmin increase the risk of serious blood clots three-fold or two-fold compared to earlier-generation oral contraceptives.
The European Medicines Agency concluded last year that such birth control pills carry a higher risk of venous thromboembolism and that warning labels should be updated accordingly.
However, it noted the overall risk of blood clot from any birth control method remains small and stopped short of advising women to stop taking pills containing drospirenone.
Yaz is the second biggest selling product made by Bayer, with $1.56 billion in global sales.