Metformin is the first-line oral medication recommended for glycemic control in patients with type 2 diabetes. We reviewed the literature to quantify the effect of metformin treatment on glycated hemoglobin (HbA_1c) levels in all types of diabetes and examine the impact of differing doses on glycemic control.

MEDLINE, EMBASE, and the Cochrane Library were searched from 1950 to June 2010 for trials of at least 12 weeks’ duration in which diabetic patients were treated with either metformin monotherapy or as an add-on therapy. Data on change in HbA_1c were pooled in a meta-analysis. Data from dose-comparison trials were separately pooled.

A total of 35 trials were identified for the main analysis and 7 for the dose-comparison analysis. Metformin monotherapy lowered HbA_1c by 1.12% (95% CI 0.92–1.32; I² = 80%) versus placebo, metformin added to oral therapy lowered HbA_1c by 0.95% (0.77–1.13; I² = 77%) versus placebo added to oral therapy, and metformin added to insulin therapy lowered HbA_1c by 0.60% (0.30–0.91; I² = 79.8%) versus insulin only. There was a significantly greater reduction in HbA_1c using higher doses of metformin compared with lower doses of metformin with no significant increase in side effects.

Evidence supports the effectiveness of metformin therapy in a clinically important lowering of HbA_1c used as monotherapy and in combination with other therapeutic agents. There is potential for using higher doses of metformin to maximize glycemic control in diabetic patients without increasing gastrointestinal effects.