RATIONALE: Estrogen can promote growth of endocrine sensitive breast cancer cells. Endocrine
therapy with tamoxifen citrate may fight breast cancer by blocking the use of estrogen by the
tumor cells. Pharmacokinetics and -genomics can have an impact on the efficacy of the
treatment.
PURPOSE: This phase III trial is studying blood samples to see if the level of active
metabolites of tamoxifen can be improved in patients with breast cancer.