Subcutaneous MYL1501D insulin glargine 100 U/mL (hereafter referred to as MYL1501D insulin glargine) [Semglee®] is a long-acting human insulin analogue approved as a biosimilar of insulin glargine 100 U/mL (hereafter referred to as reference insulin glargine 100 U/mL) [Lantus®] in various countries, including those of the EU for the treatment of diabetes mellitus in patients aged ≥ 2 years, as well as Japan for diabetes where insulin therapy is indicated. MYL1501D insulin glargine has similar physicochemical characteristics and biological properties to those of EU- and US-sourced reference insulin glargine 100 U/mL, with the bioequivalence of pharmacodynamic and pharmacokinetic parameters between these agents shown in adults with type 1 diabetes. Once-daily MYL1501D insulin glargine demonstrated noninferior glycaemic efficacy to that of once-daily reference insulin glargine 100 U/mL in adults with type 1 or 2 diabetes, with its glycated haemoglobin-lowering benefits maintained over the longer-term (52 weeks) and unaffected by previous insulin exposure. Switching between MYL1501D insulin glargine and reference insulin glargine 100 U/mL did not appear to impact glycaemic efficacy in adults with type 1 diabetes. MYL1501D insulin glargine was well tolerated, demonstrating a safety and immunogenicity profile similar to that of reference insulin glargine 100 U/mL in patients with type 1 and 2 diabetes, and in those with type 1 diabetes switching between the two agents. As expected, hypoglycaemia was the most frequently reported treatment-emergent adverse event. Thus, MYL1501D insulin glargine provides an effective biosimilar alternative for patients requiring insulin glargine therapy.