Metformin is a first-line pharmacological treatment for patients with type 2 diabetes mellitus because of its favorable overall profile, including its glucose-lowering ability, weight-neutral effects, and low risk of hypoglycemia; however, gastrointestinal (GI) intolerance may limit use in some patients. Extended-release metformin improves GI tolerability, allows once-daily dosing, and is currently available in multiple branded and generic formulations; however, it is more expensive than immediate-release metformin. Maximum plasma metformin concentrations are reached more slowly with the extended-release formulation compared with conventional immediate-release metformin, although both provide similar exposure at a given total daily dose. Extended-release metformin is as effective as immediate-release metformin in patients newly started on metformin and those switched from the immediate-release formulation, with similar weight-neutral effects. Tolerability is generally comparable, although patients switched from the immediate-release formulation--even those switched due to GI intolerance--are often better able to tolerate the extended-release formulation. Based on studies of extended-release formulations in other disease states, metformin extended-release formulation has the potential to improve patient adherence with a simpler dosing regimen and increased tolerability. Increased adherence may result in greater glycemic control, and in turn, improve outcomes and lower health care usage and costs. Extended-release metformin provides an appropriate option for patients with type 2 diabetes mellitus who require several medications to achieve glycemic control or manage comorbid conditions, and for those who have GI intolerance with the immediate-release formulation.