WASHINGTON (AP) - Progestin is the ingredient in oral contraceptive pills that provides the highest level of protection against ovarian cancer, researchers have found.

Analysis by the Duke Comprehensive Cancer Center of a 20-year-old study that showed the pill can help prevent ovarian cancer found that the cancer risk was cut by about 50 percent in all women taking pills containing the hormones estrogen and progestin.

However, women who took pills containing high levels of progestin reduced their risk of ovarian cancer by an additional 50 percent, said Patricia G. Moorman, a Duke University Medical Center researcher and the co-author of a study in the Journal of the National Cancer Institute.

"The take-home message from this study is that oral contraceptives are protective against ovarian cancer and our finding that the high progestin potency effect is a scientific (result) that might lead to new protective" drugs against ovarian cancer, Moorman said.

The study is based on a re-examination of the medical and oral contraceptive histories of more than 3,200 women who took part in a study project conducted from 1980 to 1982. The group included 390 women who developed ovarian cancer and 2,865 who did not. It compared the ovarian cancer outcome among women who did not take the pill and with women who took different formulations of the pill.

The groups included women who took no pills; those who took pills high in both estrogen and progestin; women who took pills high in one or the other of the hormones, and women who took pills with low levels of both hormones.

Moorman said earlier results had proven that the pill protects against ovarian cancer, while the new study shows which of two hormones in the pill, estrogen and progestin, are most protective.

She said the study "should lead to the investigation of progestin as a chemopreventative agent for ovarian cancer."

Moorman said the pills used by the women in the study 20 years ago are not now commonly available. She said birth control pill formulations have changed over the years as research showed that pills with lower hormone levels were effective contraceptives. Pills with lower levels of hormone generally have fewer side effects.

Ovarian cancer is the sixth most common cancer among women, excluding the skin cancers. It accounts for about 4 percent of all cancers in women, with more than 23,000 new cases diagnosed in 2001, according to the American Cancer Society.

Nearly 14,000 American women died of ovarian cancer in 2001.

Copyright 2002 The Associated Press. All rights reserved.