Tamoxifen, the celebrated drug credited with slashing breast cancer death rates worldwide, could be eclipsed by a newer medicine that is even more effective at preventing a recurrence of the disease in women whose tumors were caught early and removed.

A large, international study of postmenopausal women with early-stage cancer found that those who took tamoxifen for 2 1/2 years and then switched to exemestane for another 2 1/2 years were one-third less likely to suffer a recurrence than those who took tamoxifen the whole time.

The women switching to exemestane also had less-serious side effects, were 56 percent less likely to get cancer in the other breast, and were half as likely to develop unrelated cancer in other parts of the body.

Lead researcher Dr. R.C. Coombes, professor of cancer medicine at Imperial College School of Medicine in London, predicted doctors would give exemestane to many women at high risk for recurrence, such as those whose breast cancer had spread to multiple lymph nodes.

Exemestane, which went on the market in 1999 for advanced breast cancer, is a hormonal drug sold under the brand Aromasin. It is part of a newer class of breast cancer drugs called aromatase inhibitors.

The findings were published in today’s New England Journal of Medicine. The research was partly funded by Pfizer Inc., the maker of Aromasin.