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OBJECTIVE - 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. RESEARCH DESIGN AND METHODS - 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. RESULTS - A total of 35 trials were identified for the main analysis and 7 for the dose-comparison analysis. Metformin monotherapy lowered HbA1c by 1.12% (95% CI 0.92-1.32; I 2 = 80%) versus placebo,metformin added to oral therapy lowered HbA 1c by 0.95%(0.77-1.13; I 2 = 77%) versus placebo added to oral therapy, and metformin added to insulin therapy lowered HbA 1c by 0.60% (0.30-0.91; I 2 = 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. CONCLUSIONS - 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. © 2012 by the American Diabetes Association.

Original publication




Journal article


Diabetes Care

Publication Date





446 - 454