Comparison of Two Treatment Regimens (Sitagliptin Versus Liraglutide) on Participants Who Failed to Achieve Good Glucose Control on Metformin Alone (MK-0431-403)

Inclusion criteria * Type 2 diabetes mellitus. * On stable dose of metformin monotherapy at a dose of at least 1500 mg per day for at least 12 weeks and a hemoglobin A1C ≥7.0% and ≤11.0%. * Capable of using a liraglutide pen device. Exclusion criteria * History of Type 1 Diabetes mellitus. * Use of any oral antihyperglycemic agent (AHA) besides metformin, within the prior 12 weeks of screening. * Cardiovascular disorders within the past 3 months including acute coronary syndrome or new or worsening symptoms of coronary heart disease, coronary artery intervention, stroke, or transient ischemic neurological disorder. * Impaired liver function. * Impaired kidney function. * History of malignancy or clinically important hematological disorder that requires disease-specific treatment (chemotherapy, radiation therapy, surgery) or, in the opinion of the investigator, is likely to recur during the duration of the study. * History of leukemia, lymphoma, aplastic anemia, myeloproliferative or myelodysplastic diseases, thrombocytopenia, or malignant melanoma, regardless of the time since treatment. * Pregnancy or breastfeeding, or intention to become pregnant or donate eggs within the projected duration of the study. * Participation in another study with an investigational drug or device within 12 weeks prior to screening. * History of hypersensitivity or any contraindication to sitagliptin, liraglutide, glimepiride, or metformin based upon the labels of the country of the investigational site. * Participation in a weight loss program and not yet in maintenance phase, or starting of a weight loss medication (such as orlistat or phentermine) within the prior 8 weeks. * Surgery within the prior 4 weeks or major surgery planned during the study. * Personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia syndrome type 2 (MEN 2). * User of recreational or illicit drugs or recent history (within the last year) of drug abuse or increased alcohol consumption.