The primary finding of this current randomized, crossover, controlled clinical trial is that metformin added to insulin did not improve metabolic control including blood glucose, blood pressure, lipid profiles, and body weight, as well as glucose variability as measured with parameters obtained from CGM and insulin sensitivity as measured with lnIS among lean/normal-weight adolescents with T1D inadequately controlled with insulin. Here, INS is linked to type 1 diabetes mellitus.