Objective: The Food and Drug Administration recently updated metformin prescribing recommendations for patients with diabetes and renal disease. The American Diabetes Association as well as the American Association of Clinical Endocrinologists and American Clinical Endocrinologists also recommend periodic monitoring of vitamin B12 levels for patients using metformin. A review of the literature was conducted to assess data to evaluate the recent updates to metformin usage and provide rationales for these recommendations. Data Sources: PubMed MESH terms “Diabetes Mellitus, Type 2” and “Renal Insufficiency, Chronic” and “Metformin” were searched with an English limitation from 1990 to May 2017. A MEDLINE search was conducted using the terms “metformin” and “renal disease” from 1990 to May 2017. A PubMed search was conducted using the MESH terms “vitamin b12 deficiency” and “metformin” from 1970 to May 2017. A MEDLINE search was conducted using terms “metformin” and “vitamin B12 deficiency” with an English limitation from 1970 to May 2017. Study Selection and Data Extraction: Retrospective and prospective clinical trials, meta-analyses, and systematic reviews were considered for inclusion. Citations from identified articles were also reviewed for inclusion. Data Synthesis: The incidence of metformin-associated lactic acidosis is minimal. Data indicate metformin-treated patients with an estimated glomerular filtration rate above 30 mL/min/1.73 m2 have a reduction in mortality. Additionally, data suggest metformin may lead to vitamin B12 deficiency. Conclusion: Data support recommendations for metformin use in patients with diabetes and renal insufficiency with an estimated glomerular filtration rate above 30 mL/min/1.73 m2. Data also suggest that baseline and periodic testing of vitamin B12 levels are warranted and supported by clinical guidelines due to the risk of vitamin B12 deficiency in metformin-treated patients.