The aim of this study was to examine the use of and changes to the initial oral antidiabetic agents that are prescribed in South Korea. This study examined the health records of patients who were 30 years or older and newly diagnosed with type 2 diabetic mellitus between January 2005 and December 2009 at the Hanyang University Guri Hospital in Korea. Based on the patients’ internal medical records, this research statistically analyzed their prescription details for oral antidiabetic agents (including dates), and levels of HbA1c, fasting glucose, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and serum creatinine. Additionally, individual patients were followed up for 1 year after the initial prescription for antidiabetic agents. Any changes in drug prescriptions, including patterns and dates, were investigated and analyzed. In total, 526 patients were examined in this study. The most commonly used medication for the initial oral antidiabetic agent was metformin monotherapy (39.0 %). The proportion of patients whose prescriptions for oral antidiabetic agents changed within a year after being diagnosed with diabetes was 32.7 %, and the most common change in the treatment pattern was an add-on. Because it is not easy to determine the proper oral antidiabetic agent for individual patients during the incipient stage, it appears that finding an appropriate drug requires a change in the antidiabetic agents to achieve the therapy goal for the HbA1c and fasting glucose levels.