Prevention of type 2 diabetes mellitus (T2DM) in prediabetic patients is a pressing concern due to its increasing prevalence. The aim of this study was to evaluate the efficacy of preventive pharmacotherapy in delaying progression of incident impaired glucose tolerance (IGT) and impaired fasting glycemia (IFG) to T2DM. The participants of the study (1,136 subjects) found healthy by a regular annual checkup underwent repeat screening for T2DM. Blood samples were processed following the guidelines for good preanalytical sample preparation. Patents with incident IGT/IFG were prescribed medication therapy with metformin or/and acarbose. The rate of IGT/IFG conversion to T2DM was evaluated in years 3 and 10 of observation. Carbohydrate metabolism disorders were detected in 18.5% (n = 210) of the re-screened patients: 5.0% had T2DM, 5.5% had IGT, 8.0% had IFG. Patients with incident T2DM were prescribed blood sugar lowering therapy and they were excluded from further analysis. Patients with IGT/IFG (n = 151) were given recommendations on lifestyle modification and prescribed metformin (77%) or a combination of metformin and acarbose (23%). Three years after the start of observation, the rate of conversion to T2DM was 6.8% in patients undergoing monotherapy with metformin and 11.4% in patients undergoing combination therapy with metformin and acarbose. After the active follow-up phase was over, the majority of the patients (n = 85) decided to discontinue preventive therapy without consulting their physicians. Ten years after the active follow-up phase, the rate of NGT/IFG conversion to T2DM was 38.8% in patients who had discontinued their treatment and 0% in patients still taking metformin (р < 0.01). Long-term therapy with metformin prevented progression to T2DM in the long run in 83.3% (р < 0.05).