PurposePatients managing type 2 diabetes mellitus (T2DM) often require combination therapy to meet their blood glucose control targets. With limited real-world evidence focused on the use of glucagon-like peptide 1 receptor agonist (GLP-1RA) therapies, the objective of this study was to describe the association between semaglutide once weekly (OW) initiation and changes in hemoglobin A1c (A1c) levels. MethodsThis retrospective, descriptive cohort study used the HealthCore Integrated Research Environment (HIRE) to examine commercially insured and Medicare Advantage patients who had T2DM while taking semaglutide OW from December 1, 2017, to April 30, 2019. The first semaglutide OW prescription fill was defined as the study index date. Changes in mean A1c levels and A1c target attainment were evaluated among an intention-to-treat (ITT) population (overall group). Furthermore, a persistent population (PP) analysis on the same outcomes was performed that was limited to ITT patients who were observed to continue to use semaglutide OW at the time of the postindex A1c measurement. In addition, these outcomes were explored in patients stratified based on prior use of GLP-1RA therapy (experienced vs naive) and baseline A1c values >9% (75 mmol/mol). FindingsA total of 1888 patients were identified in the overall ITT group. The mean change in the overall ITT group between preindex and postindex A1c values was −0.9% percentage points (−9.8 mmol/mol) (mean preindex A1c, 8.2% [66.1 mmol/mol]) and −1.1 percentage points (−12.0 mmol/mol) (mean preindex A1c, 8.2% [66.1 mmol/mol]) in the PP subgroup (all P < 0.001). Among the subgroup of patients with a baseline A1c value >9% (75 mmol/mol), percentage point changes in A1c were −2.2 (−24.0 mmol/mol) and −2.4 (−26.2 mmol/mol) (all P < 0.001). When accounting for prior GLP-1RA use, the GLP-1RA–naive stratum had double the mean reduction in A1c compared with the GLP-1RA–experienced stratum (−1.2 [−13.1 mmol/mol] vs −0.6 [−6.6 mmol/mol] percentage points). ImplicationsSemaglutide OW is associated with clinically and statistically significant A1C reduction and increases in reaching A1 targets using real-world data, even in GLP-1RA–experienced patients, despite more frequent use of insulin or sodium glucose transport protein 2 inhibitors in this group. Target A1c attainment significantly increased overall and within all subgroups and strata, with approximately half of all patients attaining an A1c value <7% (53 mmol/mol) and three-quarters attaining an A1c value <8% (64 mmol/mol).