AIM: The study aimed to investigate the effect of drug-related problems (DRPs) on changes in blood glucose level (BGL) in the treatment of type 2 diabetes mellitus (T2DM) patients.METHODS: This three-month prospective cross-sectional study was conducted to patients of T2DM with complications hospitalised in Haji Adam Malik (HAM) Hospital, Medan, Indonesia period from July to October 2018. DRPs were identified and classified by using Cipolle DRP classification and trustable literature. The data obtained were analysed by Chi-Square test (p < 0.05 implied that there was a significant relationship).RESULTS: This study involved 81 T2DM patients, 52 (64.2%) of the patients were male, and 29 (35.8%) of them were female. Most (30.9%) of patients were at the age of 51-60 years. Combination of rapid-acting and long-acting insulin was the most frequently provided antidiabetic drugs (69.1%). There were 68 DRPs experienced by 32 (39.5%) of the patients. Percentage of DRP experienced by the 32 patients by number: 1 DRP, 53.1; 2 DRPs, 28.1; 3 DRPs, 3.1; 4 DRPs, 3.1; 5 DRPs, 3.1; 7 DRPs, 9.3. This study showed that 27.2% and 12.3 % of the patients had hyperglycemia and hypoglycemia, respectively. There was no significant relationship between BGL and indication without drug therapy (p = 0.064), ineffective provided drug (p = 0.079), and there was a significant relationship between BGL and irrational dose (p = 0.000). Furthermore, there was a significant relationship between hypoglycemia and adverse drug reaction (p = 0.000).CONCLUSION: DRPs are common among T2DM patients and still required the attention and appropriate actions of healthcare providers.