A study in the journal Lancet says AstraZeneca's Arimidex proved superior to standard treatment with tamoxifen for women with early stages of breast cancer. In trials, women switched from tamoxifen to Arimidex, an aromatase inhibitor, for at least three years after after first treatment and immediately after surgery. The new drug costs eight times more than tamoxifen in the U.K., however.

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