The Danish study, reported in The New England Journal of Medicine, showed that adding tamoxifen either before or after treatment with letrozole didn't improve a woman's chances of survival.
In another study, researchers at Washington University in St. Louis report a very low dose of estrogen could help women whose breast cancer has come back after treatment.
The scientists looked at women who were given low doses of estrogen after their breast cancer returned. They found the hormone actually helped slow down the progression of the disease in many patients.
The treatment wasn't always permanent, but researchers suggest it's easier to tolerate than chemotherapy. It is also considerably less expensive.
The findings are published in the Journal of the American Medical Association.