Study to Assess the Efficacy and Safety of Sitagliptin Added to the Regimen of Patients With Type 2 Diabetes Mellitus With Inadequate Glycemic Control on Metformin (0431-189)

Inclusion Criteria: * Patient Has Type 2 Diabetes Mellitus * Patient Is 30-78 Years Of Age On The Day Of Signing Informed Consent * Patient Is Currently On Metformin Therapy (1500 Mg/Day) * Patient Is A Male Or A Female Who Is Unlikely To Conceive, As Indicated By At Least One Yes Answer To The Following Questions: A) Patient Is A Male. B) Patient Is A Surgically Sterilized Female. C) Patient Is A Postmenopausal Female 45 Years Of Age With \>2 Years Since Last Menses. D) Patient Is A Non-Sterilized Premenopausal Female And Agrees To Use An Adequate Method Of Contraception To Prevent Pregnancy Throughout The Study Starting With Visit 1 And For 14 Days After The Last Dose Of Study Medication * Patient Understands The Study Procedures, The Alternative Treatments Available, The Risks Involved In The Study And Voluntarily Agrees To Participate By Giving Written Informed Consent * Patient Has An A1c of 6.5 % - 11.0% Exclusion Criteria: * Patient Has A History Of Type 1 Diabetes Mellitus Or History Of Ketoacidosis