WASHINGTON -- The Food and Drug Administration announced Thursday that it is requiring manufacturers of popular pills such as Ambien and Zolpimist that help millions of Americans sleep at night to cut the recommended dosage in half for women after laboratory studies showed that the medicine can leave patients drowsy in the morning and at risk for car accidents.
The new requirement applies to drugs containing the active ingredient zolpidem, by far the most widely used sleep aid. Using lower doses means less of the drug will remain in the blood in the morning hours and will reduce the risk that people who use the drug will be impaired while driving.
The agency told manufacturers that the recommended dosage for women, who eliminate zolpidem from their bodies more slowly than men, should be lowered from 10 milligrams to 5 milligrams for immediate-release products such as Ambien, Edluar and Zolpimist. Extended-release product dosages should be lowered from 12.5 milligrams to 6.25 milligrams, the agency said.
For men, the agency informed manufacturers that labels should recommend that health care providers should "consider" prescribing lower doses.
An estimated 10 percent to 15 percent of women will have a level of zolpidem in their blood that could impair driving eight hours after taking the pill, while only about 3 percent of men do, said Robert Temple, deputy director for clinical science in the FDA's Center for Drug Evaluation and Research.
Patients taking the higher doses should continue them for the time being, agency officials said, but should consult with their doctors about altering the dose. Doctors will still be told that they can prescribe the higher dosage if the lower one does not work, Dr. Temple said.
Sleeping pills have boomed in popularity along with the increasingly frantic pace of modern American life. According to IMS, a health care and technology company, about 60 million prescriptions for sleeping pills were dispensed in 2011, up by about 20 percent since 2006. About 40 million of those were for products containing zolpidem.
Reports of ill aftereffects from sleeping pills have circulated for years, and some doctors questioned why the FDA took so long to act. Mishaps resulting from sleepy driving -- and even strange acts of texting, eating or having sex in the night without any memory of it in the morning -- have been known for years.
"In this case, the FDA may be behind the eight ball," said Daniel Carlat, associate clinical professor of psychiatry at Boston's Tufts University. "This has been a known problem. Few doctors will be surprised hearing about this. They'll say, 'Oh yeah, we've already seen this in our patients.' "
He added that Thursday's announcement "will be good for public health because it will get patients to ask their doctors about the appropriate dosage."
Agency officials acknowledged that they have received reports of impaired driving and other problems associated with the drug for years, but said it wasn't so easy to draw a direct connection between the reports and the drug's effect. Patients often did not know what time they took their pill. Sometimes they had been drinking.
It wasn't until control trials of the drug Intermezzo, which was approved in 2011 for middle-of-the-night waking, that driving simulation tests were done on patients taking zolpidem.
Using those results, the agency began to put together a more complete picture of the risks, said Ellis Unger, a cardiologist and internal medicine specialist and an official at the FDA's Center for Drug Evaluation and Research.
The agency linked the driving simulation information with data from manufacturers on patients' blood levels and determined that levels above 50 nanograms per milliliter increased the risk of crashing while driving.
"A lot of people are wondering about the elephant in the room," Dr. Unger said. "Why did this take so long? This is science, and our thinking evolves over time."