BACKGROUND: Some patients with type 2 diabetes mellitus (T2DM) on insulin have poor glycemic control and require add-on therapy to reach target glucose values. Increased insulin doses or the addition of an oral antidiabetic drug (OAD) may improve glycemic control, but many patients fail to achieve target values. The aim of this study was to compare the treatment efficacy and safety of three different therapies in such patients. METHODS: T2DM outpatients with poor glycemic control (HbA1c ≥ 7.0%) despite insulin therapy (including patients on OADs other than a sodium-glucose cotransporter 2 (SGLT2) inhibitor) were included. The patients had a body mass index (BMI) of ≥ 22 kg/m2 and an estimated glomerular filtration rate (eGFR) of ≥ 45 mL/min/1.73 m2, did not have depletion of endogenous insulin, and had stable glucose levels for 3 months before study entry on insulin therapy. Treatment was continued for 24 weeks with insulin dose-increase therapy, tofogliflozin add-on therapy, or a combination of insulin glargine + tofogliflozin. The primary endpoints were HbA1c, weight, and total insulin dose. Secondary endpoints included fasting plasma glucose (FPG), blood pressure, lipid profiles, and incidence of adverse events. RESULTS: At baseline, the participants' median age was 59.0 years, mean BMI was 28.7 kg/m2, mean eGFR was 89.2 mL/min/1.73 m2, mean HbA1c was 8.7%, and mean FPG was 174.1 mg/dL. The mean duration of insulin therapy was approximately 7 years. The mean daily insulin dose was approximately 40 U in the three groups. Overall, 85% received other background OADs in addition to insulin. Over the 24-week period, HbA1c in the insulin group decreased slightly initially and then plateaued; daily total insulin dose and weight increased, and blood pressure increased slightly. In the insulin + tofogliflozin group and the glargine + tofogliflozin group, HbA1c decreased greatly initially, and this continued over the 24-week period, with HbA1c decreases of -1.0% and -0.8%, respectively; total daily insulin dose (-2.6 and -12.7 U, respectively) and weight (-2.9 and -3.4 kg, respectively) decreased, and blood pressure decreased slightly. Tofogliflozin therapy was well tolerated. CONCLUSIONS: Tofogliflozin may offer a new option for patients whose T2DM remains inadequately controlled on insulin therapy with or without additional oral glucose-lowering agents.