Breast cancer's sudden decrease in several countries can be credited to a 2002 federal warning against overuse of hormone-replacement drugs after menopause, a new study argued. The findings, published Wednesday in the New England Journal of Medicine, were disputed by a hormone-pill maker and others, adding to the debate over the safety of such treatment.

Before 2002, about 210,000 U.S. women each year were diagnosed with breast cancer, according to federal statistics. After 2002, the rate suddenly dropped to below 190,000...