THE US National Cancer Institute plans to test nationwide a drug called taihoxifen that might prevent breast cancer, lower the number of deaths from heart attacks and reduce the number of broken bones among middle-aged and older women. The trial, over a period of five years, will involve about 16,000 women between the ages of 35 and 59, half of who will be given a 20-milligram dose of the drug and the others a placebo.

Tamoxifen has some side effects such as blood clotting and early opause, and uterine cancer in older women. The rug has also been linked to liver cancer in rats.