The U.S. Food and Drug Administration said it "remains concerned" a certain type of birth control pill that contains drospirenone could raise the risk of blood clots beyond that of other pills.
In a drug safety communication posted on the agency's website Monday, the FDA said preliminary results of an agency funded study involving 800,000 women suggest about a 1.5-fold increase in the risk of blood clots for women who use drospirenone-containing birth control pills compared to users of other hormonal contraceptives.
Drospirenone is used in pills including Yaz and Yasmin, which are marketed by Bayer.
Drospirenone is a type of female sex hormone called a progestin. Some drospirenone pills are also available in generic versions.
But the FDA said it has not reached a final conclusion that the products raise the risk of blood clots beyond that of other pills and will convene an outside panel of medical experts on Dec. 8 to discuss the matter.
Most birth-control pills contain two types of hormones, estrogen and progestin. All types of pills increase the risk of blood clots and product labels warn of such risk.
The FDA said there's conflicting information about the blood-clot risk possibly associated with drospirenone from six other published studies. Some of the studies have reported that the risk of blood clots for women who use birth-control pills containing drospirenone is higher than that for women who use birth-control pills containing levonorgestrel, while other studies haven't found such a risk. While the risk of blood clots is low among women who take birth-control pills, the FDA said it is higher than the risk among women who aren't taking the pills.