Doctors say a newer drug is showing better results in preventing the return of breast cancer than a more traditional drug. They say the drug, Arimidex, should now be the first-choice of treatment for most women who have had the disease. Research suggests it might be able to prevent 70 to 80 percent of the most common type of tumors that occur in women after menopause. The older drug, Tamoxifen, works by blunting the effects of estrogen, a hormone that promotes the growth of about three-fourths of the tumors that occur in postmenopausal women. Arimidex prevents the production of estrogen in the first place.