Do the benefits of suppressing a woman's menstrual cycle outweigh potential risks? Scientific evidence so far suggests they may. Dissenters, however, say it's too early to tell.

While Seasonale is the first extended-cycle FDA-approved oral contraceptive on the market, it's not the first contraceptive to halt menstrual flow. Depo Provera, an injectable hormone on the market since 1992, has a similar effect for half of its users. The progestin-only injection, administered once every three months, is associated with side effects including weight gain and depression. These problems have not been reported with Seasonale.

Because Seasonale's components -- synthetic progesterone (.15 micrograms of levonorgestrel) and estrogen (.3 micrograms of ethinyl estradiol) -- are the same as in Nordette, an older birth control pill, the FDA required the product undergo only one year of a clinical trial, instead of two (required for pills with new components). The trial, which involved about 1,400 women between the ages of 18 and 40, found risks and benefits similar to those associated with traditional oral contraceptives.

Birth control pills, on the market since the 1960s, are currently used by more than 18 million women, according to the National Institututes of Health.

The pill has been shown to reduce risk for some conditions, including ovarian and endometrial cancer. Studies have also shown that they can increase a woman's risk for other diseases, such as strokes and liver cancer.

But Seasonale delivers nine more weeks' worth of hormones per year than Nordette, because the new product follows a 84/7 regimen (84 days on the pill, followed by 7 days off) instead of Nordette's 21/7 schedule. For that reason, some health professionals say more long-term safety data is needed.

"The first rule of medicine is do no harm," said Lawrence Nelson, head of the National Institute of Child Health and Human Development's unit on gynecologic endocrinology. "We are talking about giving more of a drug to go beyond protecting from pregnancy to remove menstrual cycle out of convenience. This may have adverse effects we don't know about."

Susan Rako, a Massachusetts psychiatrist who wrote the book "No More Periods? The Risks of Menstrual Suppression" (Harmony Books, 2003), calls Seasonale "an uncontrollable experiment," all of whose effects won't be known for at least 30 years.

Barr Pharmaceuticals -- and many ob-gyns -- dispute that assessment. "The chemical compounds we are using are at least 25 years old; they have been tested and are tried and true," said Carole Ben-Maimon, president of research for Barr.

Here is a summary of the pros and cons of using Seasonale.

Pros

* Convenience: four periods a year, instead of the usual 13.

* Nearly 100 percent effective at preventing pregnancy when used as directed.

* Reduced risks of anemia (low iron), seizures, fibroids, menstrual migraines.

* Decreased risk of -- and pain from -- endometriosis (an abnormal growth of the tissue lining the uterus, affecting 5.5 percent of U.S. women).

* Potential lower risk of uterine cancer (exacerbated by endometriosis and irritation of the uterine lining) as well as ovarian cancer (by cutting down the number of a woman's cumulative periods).

* Seasonale has a safety profile comparable with that of traditional oral contraceptives.

* Fewer cramps and mood swings associated with premenstrual syndrome (PMS).

* Savings estimated at up to $41 per month (for sanitary protection, pain relievers, lost wages and doctor visits associated with menstrual pain).

* Fertility and the resumption of menstruation and ovulation are not affected by Seasonale.

Cons

* 63 additional days of hormones.

* Spotting and bleeding between periods, especially during the first few cycles.

* Increase in risks of strokes, heart attacks and blood clots as well as breast, cervical and liver cancers, similar to that associated with other oral contraceptives.

* Potential to mask menstrual complications such as ovarian failure or amenorrhea.

* Menstruation can't be used as a pregnancy indicator. Women who think they may be pregnant while using Seasonale should take a pregnancy test.

* Like other oral contraceptives, Seasonale is not advised for women who smoke, have had a heart attack, blood clots, certain cancer or liver diseases, unexplained vaginal bleeding or those who might be pregnant.

* No data on effects on teenagers. An estimated 1.2 million girls ages 15-19 take oral contraceptives. The Seasonale trial -- as is standard for clinical trials, given consent issues -- included no participants under age 18.

* Limited longitudinal research.

-- Elizabeth Gettelman