The drug tamoxifen, already widely given to older breast cancer patients, works equally well in young women and could save 20,000 more lives a year worldwide, according to a study published Friday.

The research, which concluded that the drug is more powerful than scientists have believed, is described as the largest cancer study ever undertaken.The study - published in this week's issue of the Lancet, the British medical journal - found that when given immediately after surgery for five years, tamoxifen cut in half the recurrence rate over the next decade - regardless of age. The drug also slashed in half the chances of a woman developing a new cancer in her other breast.

"The evidence is getting to be quite strong that these are permanent preventions - in other words, a cure," said Dr. Harmon Eyre, chief medical officer of the American Cancer Society. "This takes it beyond any question of scientific debate."

The study by the Early Breast Cancer Trialists' Collaborative Group was headed by scientists at the University of Oxford. It is part of an ongoing analysis of all tamoxifen studies ever conducted worldwide and followed 37,000 women with early breast cancer. It was funded by the Imperial Cancer Research Fund, an independent charity.

The detection of breast cancer is considered early if the growth is spotted only in or near the breast and can be removed by surgery. However, undetectable fragments may already have spread elsewhere.

In most cases, cancer responds to hormones that encourage it to grow. Tamoxifen attacks spreading cancer by blocking these hormones. The drug does not help when tumors are not sensitive to hormones.

With 1 million women worldwide taking it, tamoxifen is one of the most widely used cancer drugs. But it is most often given to post-menopausal women. Younger women have been mostly treated with chemotherapy alone because doctors did not believe tamoxifen helped, theorizing that high hormone levels in pre-menopausal women might overwhelm the drug.