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Women who take estrogen after menopause have a 46 percentgreater risk of breast cancer, according to a new report in Thursday's New England Journal of Medicine. An increasing number of doctors are giving the hormone to older women because ofits ability to strengthen bones and ward off heart trouble. But the new findings suggest such benefits may come at a steep price. Several earlier studies had indicated a link between estrogen and breast cancer, butthis is the largest survey yet, based on results from the Nurses Health Study, which has followed 121,700 women nurses since 1972. While the new findings are significant, TIME science reporter Alice Park says they're far from definitive. "It's not the best study, mainly because it's not a true random sample of the population. So there could be factors other than estrogen involved." A definitive, randomized survey is in the works. But the results won't be known until 2009.