Patient description: The patient is a 24-year-old man who has had type 1 diabetes for 11 years. He presents to the emergency room with hyperglycemia and concern for possible diabetic ketoacidosis after not taking his insulin for 3 days. The patient reports that he is currently homeless and has lost his supply of insulin, syringes, glucometer, and glucose testing supplies. The patient states that at the time of his initial diagnosis with type 1 diabetes he was hospitalized with a glucose value >1000 mg/dL. At the time, he was experiencing polyuria, polydipsia, and polyphagia. He reports that he has been on insulin since the time of his diagnosis, and he has never been prescribed oral agents for diabetes management. Most recently, he has been using insulin glargine 55 units once daily, and insulin aspart sliding scale 3 times daily. The patient has had previous episodes of diabetic ketoacidosis, for which he was hospitalized. With this episode of hyperglycemia, he is not experiencing any nausea, vomiting, or abdominal discomfort, and he appears well. His lab studies showed: A1c: 11.3% Creatinine: 0.9 mg/dL with eGFR >60 mL/min Aspartate aminotransferase (AST): 17 U/L Alanine aminotransferase (ALT): 14 U/L Beta-hydroxybutyrate: 0.1 mmol/L Bicarbonate: 25 mEq/L Anion

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Diabetes Diabetes Mellitus, Type 1 Diabetes Mellitus, Type 2 Delivery Systems Informed consent obtained before any trial-related activities Subjects diagnosed with type 1 or type 2 diabetes. If type 2 diabetics, treatment with or without oral anti diabetic medication is allowed Current users of vial/syringe (pen naïve) treated with short-acting insulin (insulin aspart, glulisine or lispro) and once daily long-acting insulin (detemir or glargine) or once daily long-acting insulin (detemir or glargine) alone Treatment with insulin (i.e. aspart, glulisine, lispro, detemir or glargine) for at least 6 months Body Mass Index (BMI) less than 45.0 kg/m^2 HbA1c less than or equal to 9.0% at screening visit based on analysis from central laboratory Able and willing to adhere to the trial-specific insulin regimen for the entire trial period  Females of childbearing potential who are pregnant, breast-feeding or intend to become pregnant or inadequate contraceptive techniques during the trial period (adequate contraceptive measures are considered as intrauterine device, oral contraceptives and barrier methods) Previous participation in this trial (screening visit) Systemic drugs that may influence glycaemic control (e.g., corticosteroids) Known or suspected allergy to trial product(s) or related products Known or suspected abuse of alcohol or drug abuse Mental incapacity, unwillingness or language barriers precluding adequate understanding or cooperation Previous treatment with sitagliptin Clinically significant, active (or over the past 12 months) disease of the gastrointestinal, neurological, genitourinary, or haematological systems Cardiac disease defined as: Decompensated heart failure (New York Heart class III or IV, unstable angina pectoris within the past 6 months of study enrolment, myocardial infarction within the past 12 months and a clinically significant history of arrhythmias or conduction delays on electrocardiogram (ECG) over the past 12 months Any other severe acute or chronic illness as judged by the Investigator