Labels for new varieties of birth control pills should include its association with a higher risk of blood clots in the legs and lung than with older contraceptive pills, the Food and Drug Administration reports.

In both cases, actual occurrences of blood clots are rare. It is estimated that 10 in 10,000 women taking the newer birth control pills will develop a blood clot. But the association is important because blood clots can trigger heart attacks and strokes.

Current labels for Yaz and other widely used drugs by German drugmaker Bayer are inadequate, an FDA panel of experts found.

"Clearly the wording is inadequate and incomplete," said Dr. Richard Bockman of New York's Hospital for Special Surgery. "Adverse events have to be made graphic so physicians and patients are aware of the consequences."

The pills in question use a synthetic hormone called drospirenone, meant to mimic the effect of the female hormone progesterone.

But a handful of large studies since 2009 show the drospirenone pills carry a higher risk of blood clots than earlier birth control pills -- the most recent study indicating that women taking Yasmin (the precursor to Yaz) as 75 percent more likely to have a blood clot than women taking a combination of the older drugs.

Yaz became the best-selling birth control pill in the U.S. in 2009, but prescriptions have nose-dived since then amid safety concerns. Thousands of personal injury lawsuits are pending against Bayer.