OBJECTIVES:  To establish the effectiveness of antidiabetic therapy and the frequency of clinical inertia in the management of type-2 diabetes mellitus in Colombia - 2015. METHODS:  A cross-sectional retrospective study with follow-up was conducted in 23 cities in diabetic patients who had been treated for at least one year and were receiving medical consultation for antidiabetic treatment. Effectiveness was established when haemoglobin A1c levels were <7% in general population and <8% in special situations. When clinical inertia was reached, which was defined as no therapeutic modifications despite not achieving management controls. Sociodemographic, clinical, and pharmacological variables were evaluated, and multivariate analyses were performed. The study received bioethical approval. RESULTS: In total, 363 patients with type-2 diabetes mellitus were evaluated, with a mean age of 62.0±12.2 years. A total of 1,016 consultations were evaluated, and the therapy was effective at the end of the follow-up in 57.9% of cases. Clinical inertia was found in 56.8% of patients who did not have metabolic control. The most frequently prescribed medications were metformin (84.0%), glibenclamide (23.4%), and insulin glargine (20.7%). Moreover, 57.6% of the patients were treated with two or more antidiabetic medications. In the logistic regression analysis having metabolic control in the first consult of the follow-up was a protective factor against clinical inertia in the subsequent consultations (OR:0.09; 95%CI:0.04–0.17; p<0.001). CONCLUSIONS: The effectiveness of treatment for patients with type-2 diabetes mellitus has increased in Colombia, and for the first time, clinical inertia was identifiable and quantifiable and found in similar proportions to other countries with previous reports. Clinical inertia is a relevant condition given that it interferes with the possibility of controlling this pathology.