A popular new generation of birth control pills should carry stronger warnings about the risks they pose of potentially life-threatening blood clots, federal advisers recommended Thursday.

After a day-long hearing, two Food and Drug Administration advisory committees, meeting together, voted 15 to 11 that the benefits of the pills in preventing unwanted pregnancies outweighed the risks. But the panels voted 21 to 5 that the current labels fail to adequately warn of the dangers, and they urged the agency to require stronger admonitions.

The FDA does not have to follow the recommendations of its advisory panels, but it usually does.

The hearing was convened because of rising concerns about the safety of widely prescribed birth control pills such as Yaz. Studies indicate that women taking the pills face a greater risk of suffering potentially life-threatening blood clots than those taking older versions of oral contraceptives.

Yaz and several other pills, including Yasmin, Beyaz and Safyral, contain a synthetic version of the hormone progestin called drospirenone that was thought to have fewer side effects. Bayer, which makes Yaz and Yasmin, has heavily promoted the pills as offering advantages over standard birth control pills, including alleviating acne and preventing premenstrual symptoms such as moodiness, irritability, bloating, fatigue and headaches. In 2009, the FDA found those claims to be exaggerated and asked Bayer to run corrective television advertisements.

After Yaz was introduced in 2006, it quickly became the best-selling birth control pill, with U.S. sales topping $781 milion in 2009, according to IMS Health, which tracks pharmaceutical sales. Sales, however, have dropped as safety concerns have emerged, falling to $374 million in 2010. Currently, it is the fourth most popular birth control pill.

Some studies have indicated that the pills may pose a greater risk of blood clots, which can cause heart attacks, strokes and potentially fatal blockages of an artery in the lung. Most recently, the FDA analyzed the medical records of more than 800,000 U.S. women and found that for women taking the pills the risk of clots was about 74 percent higher than for women taking no contraceptives. Thousands of women are suing Bayer.

Bayer maintains that the pills are safe and that its own studies found no increased risk of blood clots.

Several consumer and women’s health groups recommended that the pills be removed from the market and were disappointed by Thursday’s outcome.

“These advisers told the FDA that preventing pregnancy is worth an increased risk of blood clots,” said Cynthia A. Pearson of the National Women’s Health Network. “We believe that women have a good range of safer contraceptive choices that don’t put them at increased risk of life-threatening complications.”

Thursday’s recommendation came one day after Health and Human Services Secretary Kathleen Sebelius overruled a decision by FDA Commissioner Margaret A. Hamburg to make the controversial emergency contraceptive Plan B available to anyone of any age without a prescription.