THE first birth control pill meant to stop women's monthly periods won approval today.

Lybrel is the first pill to receive US federal Food and Drug Administration approval for continuous use.

When taken daily, the pill can halt women's menstrual periods indefinitely as well as prevent pregnancies, its manufacturer Wyeth said.

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

But an FDA official warned the pill would not suit all women and about half of those enrolled in studies of Lybrel dropped out before the end.

Dr Daniel Shames, deputy director of the FDA's drugs office, said 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," Dr Shames said.

Wyeth, which is based in New Jersey, plans to start Lybrel sales in July.

The price of the 28-pill packs has not been established.

The pill contains a low dose of two hormones already used in birth control pills, ethinyl estradiol and levonorgestrel.

A study showed Lybrel was just as effective in preventing pregnancy as a traditional pill called Alesse, which is also made by Wyeth.

But it may be difficult for the women to recognise if they have become pregnant because Lybrel users will not have regular periods.

In one test, 59% 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 a third of all the women originally enrolled, Dr Shames said.

"Women who use Lybrel would not have a scheduled menstrual period, but will most likely have unscheduled bleeding or spotting."

He said the bleeding can last four to five days and may persist for a year.

Women who take other low-dose pills have reported similar issues.

But Dr Vanessa Cullins, vice-president for medical affairs at Planned Parenthood Federation of America Inc, said Lybrel was a welcome breakthrough.