This randomized phase III trial studies metformin hydrochloride to see how well it works
compared to placebo in preventing breast cancer in patients with atypical hyperplasia or in
situ breast cancer. Chemoprevention is the use of certain drugs to keep cancer from forming.
The use of metformin hydrochloride may prevent breast cancer.
Atypical Ductal Breast Hyperplasia, BRCA1 Mutation Carrier, BRCA2 Mutation Carrier, Ductal Breast Carcinoma in Situ, Lobular Breast Carcinoma in Situ