Objectives <p>This study aimed to evaluate the longitudinal association between overbasalization and glycemic control.</p> Methods <p>Retrospective cohort study including subjects with T2DM diagnosis, insulin users and treated in the Endocrinology Service of EsSalud through the National Telemedicine Center, part of the Social Security (EsSalud) in Lima, Peru. Data was taken from January 2023 to December 2024, and only those with at least two assessments were considered. Hospitalized participants, chronic users of corticosteroids, those with transfusions, and pregnant women during the study period were excluded. The dependent variable was glycemic control, defined using glycated hemoglobin (HbA1c &lt; 53 mmol/mol [&lt; 7%] or ≥ 53mmol/mol [≥ 7%]), while the independent variable was overbasalization, defined as a basal insulin dose &gt; 0.5 IU/Kg/day. The association of interest was evaluated cross-sectionally and longitudinally.</p> Results <p>A total of 398 records, mean age of 63.8 (SD: 10.4) years and 50.8% women, was analyzed. The mean duration of diabetes was 15.8 (SD: 8.7) years, whereas the median duration of insulin use was 3.3 (interquartile range: 6) years. The prevalence of overbasalization at the cohort baseline was 43.4%, whereas only 14.5% had appropriate glycemic control. In cross-sectional analysis, no association of overbasalization with glycemic control was found; however, in the longitudinal analysis (mean: 9.6 [SD: 3.5] months of follow-up), the group with overbasalization had a 42% reduction (95% CI: 20% − 58%) in the probability of having appropriate glycemic control.</p> Conclusions <p>Among insulin users, overbasalization was observed to be associated with reduced glycemic control and higher HbA1c levels. More than 40% of insulin users had overbasalization at the cohort baseline. Our study suggests that overbasalization is frequent among people using insulin and should be evaluated to optimize glycemic control.</p>