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

* PRE-REGISTRATION-INCLUSION CRITERIA * Must be at increased risk for breast cancer, defined as at least one of the following four criteria: * Having had a prior biopsy demonstrating atypical hyperplasia, lobular carcinoma in situ (LCIS), or ductal carcinoma in situ (DCIS) * A Gail Model Risk of \>= 1.66% over 5 years * A strong family history of breast and/or ovarian cancer which is defined as at least one of the following: * One first-degree relative with breast cancer before the age of 50 years * One first degree relative with bilateral breast cancer * Two or more first-degree relatives with breast cancer * One first degree relative and two or more second or third degree relatives with breast cancer * One first-degree relative with breast cancer and one or more relatives with ovarian cancer * Two second or third degree relatives with either breast cancer and one or more with ovarian cancer * One second or third degree relative with breast cancer and two or more with ovarian cancer * Three or more second or third degree relatives with breast cancer * Known breast cancer (BRCA)1 or BRCA2 mutation carrier providing that the woman has * Met with a genetic counselor to review genetic testing results, and * Has been offered the opportunity to undergo prophylactic mastectomy and oophorectomy * Pre-menopausal women as defined as four menstrual cycles within the last six months prior to pre-registration; women with less than 4 mens…