Objective To test the efficacy and tolerability of metformin.

Research Design and Methods An 8-mo double-blind placebo-controlled parallel-group trial was performed at University hospital diabetic clinics on 60 patients with non-insulin-dependent diabetes mellitus (NIDDM) treated by diet alone. Metformin was administered and built up to a maximum dosage of 1 g three times daily.

Results Mean HbA₁ fell from 11.7 ± 0.4 to 10.3 ± 0.4% (means ± SE) on metformin but rose from 11.8 ± 0.4 to 13.3 ± 0.4% on placebo (P <0.001). Final mean fasting blood glucose was 5.1 mM lower with metformin than placebo (P <0.001). No other biochemical variable differed significantly, and weight did not change. A favorable glycemic response was not restricted to the obese. The mean final dosage of metformin was 1.7 ± 0.1 g and was well tolerated.

Conclusions Metformin achieved a 23% lower mean HbA₁ than placebo without weight gain or significant unwanted effects.