WASHINGTON -(Dow Jones)- The U.S. Food and Drug Administration said Monday 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, 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, which is used in pills including Yaz and Yasmin, which are marketed by Bayer AG (BAYRY, BAYN.XE). Drospirenone is a type of female sex hormone called a progestin. Some drospirenone pills are also available in generic versions.

But 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.

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.

Blood clots form inside a vein and are known as deep vein thrombosis, or DVT. The clots usually form in the lower leg or thigh, but can break loose and travel to other areas of the body such as the lungs, where they are called a pulmonary embolism, or PE. The FDA said the symptoms of a DVT include the new onset of persistent leg pain, while those of a PE include severe chest pain and sudden shortness of breath.

Bayer has said its analysis of the available scientific evidence shows that the risk of developing a blood clot associated with taking pills containing drospirenone "is comparable" to that of other birth-control pills studied but is working with the FDA on the matter. Sales of Bayer's Yaz product family have been declining, which the company blames on generic competition in the U.S.

In May, the European Medicines Agency said the risk of developing blood clots for oral contraceptives containing drospirenone was higher than that of pills containing levonorgestrel but said the risk for any type of birth-control pill was small. EMA said the product labels of birth-control pills containing drospirenone will be updated.

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