A randomized , double-blind , placebo-controlled study was conducted over 12 weeks , during which 25 patients with T2DM completed treatment with either sitagliptin ( 100mg once daily ) or placebo as add-on therapy to metformin [ sitagliptin group ( n = 12 ) : meanstandard error of the mean ( s.e.m. ) age 542.5 years , means.e.m .