Limitations of our study include reliance on medication purchase data for medication use; a short study duration, limiting longer-term assessment of the metformin-cancer association; limited data on some risk factors for prostate cancer, such as the use of clinic locality to determine socioeconomic status; and lack of information on other known risk factors for prostate cancer—namely family history of prostate cancer, family history of breast/ovarian cancer linked to the breast cancer 1 gene (BRCA1) and breast cancer 2 gene (BRCA2) mutations, and obesity. This evidence concerns the gene BRCA1 and Familial prostate cancer.