Non-estrogen morning-after pill works best.

In the 1970s, Canadian scientist Albert A. Yuzpe discovered that a multiple dose of birth control pills taken within a few days of sexual intercourse could avert pregnancy by flooding a woman's system with a burst of hormones.

The so-called morning-after pill containing estrogen and progestin works about three-fourths of the time, but it often causes nausea and vomiting. Researchers now report that a pill without estrogen but with a larger dose of progestin, also called progesterone, prevents conception better than the dual pill and causes fewer side effects.

A 14-country study by the World Health Organization (WHO) tested the two kinds of pills on 1,955 women, average age 27, who reported to clinics after having unprotected sex. About half were given the two-hormone pill and a follow-up dose 12 hours later. The other women received similarly timed doses of a synthetic progestin, also called levonorgestrel, but no estrogen.

Pregnancy occurred despite the treatment in 10 of the 976 women who had received levonorgestrel and in 28 of the 979 who had been given the dual contraceptive. The researchers estimated the number of pregnancies that might have been expected in these groups, given the timing of the unprotected sex in the menstrual cycle. Levonorgestrel averted 85 percent of the potential pregnancies, while the dual pill prevented 57 percent.

Only 23 percent of the women receiving levonorgestrel reported nausea, and less than 6 percent vomited. In the other group, 51 percent had nausea and 19 percent vomited.

"Estrogen is a well-known cause of nausea," says obstetric gynecologist Felicia H. Stewart of the Henry J. Kaiser Family Foundation in Menlo Park, Calif. The milder side effects of levonorgestrel and its high pregnancy-prevention rate make the WHO findings "very persuasive," she says.

Scientists had been concerned that levonorgestrel taken by itself would disrupt a woman's subsequent menstrual cycles. This study showed no difference between the two pills in this regard, says study coauthor Paul F.A. Van Look, an obstetric gynecologist at WHO in Geneva.

Even as scientists fine-tune the emergency contraception pill, the misnomer attached to Yuzpe's discovery still stands as a classic in medical science. The morning-after pill can in fact work up to 72 hours after intercourse. A U.S. study now finds that this mislabeling has had consequences.

Stewart and her colleagues report in the August Archives of Pediatric and Adolescent Medicine that U.S. teens show scant knowledge of emergency contraception. Of 1,510 boys and girls ages 12 to 18, surveyed by telephone in 1996, only 23 percent knew that such pills exist. Of those aware of the treatment, just 9 percent understood that the window of opportunity lasts a full 72 hours.

"Many people got the misconception that anything after [the morning after] would be too late," says study coauthor Suzanne F. Delbanco, a health policy analyst currently at the University of California, Berkeley. In the United States, the pills are mostly made available at emergency rooms and college health centers for rape victims or other women who report unprotected sex, she says.

U.S. drug companies have shied away from marketing hormone pills in emergency-contraception packets in part because birth control is politically sensitive and because contraception is often not covered by medical insurance, Stewart says. But some firms now have plans to package and sell them as such.

In Seattle, a pilot project in which pharmacists could dispense emergency-contraception pills after taking a brief patient history resulted in 800 treatments in the first month, she reports.

These pills, which use hormones to prevent conception, differ from the so-called abortion pill RU-486 that blocks the hormones and can work beyond the 72-hour window.