Washington - The first birth-control pill meant to put a stop to women's monthly periods indefinitely won federal approval Tuesday. Called Lybrel, it's the first such pill to receive Food and Drug Administration approval for continuous use. When taken daily, the pill can halt women's menstrual periods indefinitely and prevent pregnancies.

Lybrel is the latest approved oral contraceptive to depart from the 21-days-on, seven-days-off regimen that had been standard since birth-control pill sales began in the 1960s. The pill, manufactured by Wyeth, is the first designed to put off periods altogether when taken without break.

The pill isn't for everyone, an FDA official said. About half the women enrolled in studies of Lybrel dropped out, said Dr. Daniel Shames, a deputy director in the FDA's drugs office. Many did so because of the irregular and unscheduled bleeding and spotting that can replace scheduled menstruation.

"If you think you don't want to go down this road, this is not for you," Shames told reporters.

Wyeth plans to start Lybrel sales in July. The Madison, N.J., company said it hasn't yet determined a price for the 28-pill packs. The pill contains a low dose of two hormones already widely used in birth-control pills, ethinyl estradiol and levonorgestrel.

A study showed Lybrel was just as effective in preventing pregnancy as a traditional pill, Alesse, also made by Wyeth. However, since Lybrel users will eliminate their regular periods, it may be difficult for them to recognize if they have become pregnant, Shames said.

Most of the roughly 12 million American women who take birth-control pills do so to prevent pregnancy. Others rely on hormonal contraceptives to curb acne or regulate their monthly periods.

Some nontraditional pills such as Yaz and Loestrin 24 shorten monthly periods to three days or less. Seasonique, an updated version of Seasonale, reduces them to four times a year. With Lybrel, in one test, 59 percent of the women who took Lybrel for a year had no bleeding or spotting during the last month of the study. However, because of dropouts, that translates into only about one-third of all the women originally enrolled in the study, Shames said.

"Women who use Lybrel would not have a scheduled menstrual period, but will most likely have unplanned, breakthrough, unscheduled bleeding or spotting," Shames said. The bleeding can last four to five days and may persist for a year, he later added. Women who take other low-dose pills have reported similar issues.

Still, a women's health expert said Lybrel would be a welcome addition for the woman who seeks relief from the headaches, tender breasts, cramps and nausea that can accompany monthly periods. Whether Lybrel relieves those symptoms was not directly studied.

"Over time she will experience markedly less bleeding episodes or no bleeding episodes," said Dr. Vanessa Cullins, vice president for medical affairs at Planned Parenthood Federation of America Inc. "That is very beneficial for some women, and is wanted by some women."

University of New Hampshire sociologist Jean Elson pointed to advantages for what she characterized as a small number of women who suffer extraordinarily during menstruation, but overall she said the pill left her with mixed feelings.

"For women in that situation, I certainly can understand the benefits of taking these kinds of medications, but for most women menstruation is a normal life event, not a medical condition," said Elson, who researches the sociology of gender and medical sociology. "Why medicate away a normal life event if we're not sure of the long-term effects?"

In recent years, as the hormone content of birth-control pills has dipped, failure rates have climbed. The FDA is considering whether to establish an acceptable failure rate for the pills. In January, a panel of agency advisers said less-effective birth-control pills should still merit federal approval if they promise other benefits, including improved safety.

Generally, lower-dose birth-control pills can reduce the risk of serious and sometimes deadly side effects, including blood clots and stroke, associated with their use.

The injectable hormonal contraceptive Depo-Provera also can eliminate monthly periods.