Introduction: Biosimilar insulins have the potential to increase access to treatment among patients with diabetes mellitus (DM), reduce treatment costs, and expand market competition. There are no published studies evaluating the performance of biosimilar insulins in routine clinical practice in Asia. This study assessed the safety and effectiveness of biphasic isophane insulin injection in Malaysian DM patients. Materials and Methods: In this open label, single-arm, observational, post marketing study, patients received biphasic isophane insulin injection as per the Prescribing Information; and were assessed for safety (adverse events including hypoglycaemia), effectiveness (glycosylated haemoglobin [HbA1c]; fasting blood sugar, [FBS]; and patient’s condition by patient and physician) over a period of 24 weeks. Results: Adult male and female diabetes patients (N=119; type 2 DM, n=117) with a mean (SD) diabetes duration of 13 years were included. No new safety signals have been identified. Significant reduction in HbA1c was observed at weeks 12 and 24 (mean [SD] - baseline: 9.6% [1.9]; Week 12: 9.0% [1.7] and at Week 24: 9.1% [1.7]; p < 0.001). There were 10 serious and 9 non-serious adverse events reported in the study. Expected mild events included hypoglycaemia and injection site pruritus. However, the majority of the adverse events were non-study drug related events. No deaths were reported during the study. Discussion: Biphasic isophane insulin injection was well tolerated with no new safety concerns. It was found effective in post- marketing studies conducted in routine clinical settings when administered in DM patients in this study.