Metformin Hydrochloride in Preventing Breast Cancer in Patients With Atypical Hyperplasia or In Situ Breast Cancer

I. Test for the presence or absence of cytological atypia (as measured by a Masood Cytology Index Score of 14-17) in unilateral or bilateral random periareolar fine needle aspiration (RPFNA) aspirates after 12 and 24 months (24 month is optional for placebo-only group for patients who remain on placebo arm and will not receive metformin \[metformin hydrochloride\]) for women receiving metformin versus placebo control. The presence of cytological atypia means any atypia in any RPFNA specimen. SECONDARY OBJECTIVES: I. Use the Masood Cytology Index Score to test for the presence of cytological atypia or disappearance of cytological atypia in RPFNA aspirates after 12 months for both arms, and 24 months (24 month is optional for placebo-only group for patients who remain on placebo arm and will not receive metformin, and mandatory for crossover patients) for women receiving metformin 850 mg orally (PO) twice daily (BID) (metformin group). II. Compare Masood Cytology Score values at 0 and 12 months in right and left breasts (if both breasts were aspirated) from the same individual in the metformin and placebo group. III. Test the reproducibility of reverse phase protein microarray (RPPM) in duplicate RPPM determinations from individual RPFNA specimens. IV. Correlate baseline RPPM values with presence of atypia (as measured by Masood Cytology Index Score) at month 12 and month 24 (month 24 optional for placebo-only group; for patients who remain on placebo arm and will not receive metformin) RPFNA. TERTIARY OBJECTIVES: I. Test whether metformin alters RPFNA or blood biomarkers associated with breast cancer risk. II. Test whether metformin alters markers associated with obesity and insulin resistance. III. Test other exploratory measures in RPFNA and serum including metformin levels and estrogen/progesterone. IV. Banking: As part of ongoing research for Alliance Cancer Control studies, banking residual blood and RPFNA products for future studies.