Background: Patients with type 2 diabetes are often obese and require large dose of insulin to achieve glycemic control. Insulin therapy often cause weight gain and results in increasing insulin requirements. This study was conducted to evaluate the efficacy of metformin in combination with insulin in patients with type 2 diabetes poorly controlled with insulin therapy alone.

Materials and Methods: In a randomized clinical trial in outpatient diabetes clinic at university medical center, 39 patients with poorly controlled type 2 diabetes who were receiving insulin therapy were randomly assigned to receive metformin in combination with inslin or insulin alone for six months.

Results: Hemoglobin A1C levels decreased by 1.56 percentage (95% cl, 0.77 to 2.34 percentage point) in metformin group, a significantly greater change (P=0.008) than the increase of 0.03 percentage points in the insulin alone group. Average final hemoglobin A1C levels were 7.96% in the metformin group and 10.32% in the insulin alone group (difference 17%). For patients who did not received metformin, the insulin dose increased 9.37 units (95% cl 6 to 12 units) or 37% more than did the dose for patients who received metformin. (P<0.0005): for these patients, the insulin dose decreased 6.6 units (95% cl-1.24 to –11.96) Patients in insulin alone group gained an average of 3.08 kg of body weight (95% cl 1.56 to 4.58 kg) patient in the metformin group gained an average of 1.8 kg (95% cl 0.21 to 3.36 kg) Difference (1.9%) was not meaningful. Change of total cholesterol, low density lipoprotein cholestrol and triglyceride levels was not significant.

Conclusion: The addition of metformin to insulin therapy resulted in hemoglobin A1C concentrations that were 17.8% lower than those achieved by insulin therapy alone. This improvement occurred with use of 37% less insulin. Metformin is an effective adjunct to insulin therapy in patients with type 2 diabetes.