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Abstract Background Elranatamab is the first bispecific B-cell maturation antigen (BCMA)-CD3–directed monoclonal antibody approved in Japan for the treatment of patients with multiple myeloma (MM) who have received at least 3 standard therapies, including proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), and anti-CD38 monoclonal antibodies (mAbs). Elranatamab is initiated through a process, which includes administration of 12 mg on day 1, 32 mg on day 4, and 76 mg on day 8. This study leveraged Japan Medical Data Vision (MDV), a Japanese hospital claims database, to describe the real-world SUD process for patients initiating elranatamab in Japan, including patient characteristics, elranatamab administration, and experience and management of adverse events (AEs) including cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS). Methods This is a retrospective observational cohort study. Adult patients with a diagnosis of MM who first received elranatamab from March 26, 2024, to March 31, 2025, outside of clinical trials, were included in the study population. Patients were grouped based on their SUD patterns, aligned with label guidance. Because actual administered doses are not recorded in claims data, vial size served as a proxy (eg, use of a 44 mg/1.1 mL vial indicated either a 12 or 32 mg administration). Patients with complete and timely SUD (Group A) were defined as those who received two 44 mg/1.1 mL vials and 1 subsequent 76 mg/1.9 mL vial, with 1 to 7 days between vials. Patients with complete but not timely SUD (Group B) were similar to Group A patients but had ≥1 interval of 8 to 14 days between vials. Results, including baseline demographics and clinical characteristics, elranatamab vial size and timing, and supportive medication use, were reported descriptively for Groups A and A+B during the SUD period (ie, from first 44 mg/1.1 mL vial to first 76 mg/1.9 mL vial). CRS and ICANS were identified using specific disease codes within 14 days following elranatamab initiation. Results A total of 253 patients initiated elranatamab and were included in the overall study population; 218 (86.2%) had a complete SUD period (Group A+B: Group A: 193 [88.5%]; Group B: 25 [11.5%]). The mean (SD) time from the first 44 mg/1.1 mL vial to the first 76 mg/1.9 mL vial was 8.3 (2.2) days and 9.0 (2.8) days in Groups A and A+B, respectively. Alternative SUD patterns were observed in 9 (3.6%) patients who received ≥1 44 mg/1.1 mL vial followed by a 76 mg/1.9 mL vial; the remaining 26 (10.3%) patients did not receive both vial sizes. The mean (SD) age was 73.1 years (8.3) in Group A and 72.6 years (8.6) in Group A+B. Females comprised 55.4% of Group A and 55.0% of Group A+B. Most patients received ≥1 PI (Group A: 170 [88.1%], Group A+B: 194 [89.0%]); ≥1 IMiD (Group A: 180 [93.3%], Group A+B: 204 [93.6%]); and ≥1 anti-CD38 mAb (Group A: 178 [92.2], Group A+B: 202 [92.7%]) before elranatamab initiation. Fewer patients had previously received chemotherapies (Group A: 97 [50.3%], Group A+B: 113 [51.8%]) or a stem-cell transplant (Group A: 29 [15.0%], Group A+B: 35 [16.1%]). CRS events were identified in 91 (47.2%) patients in Group A and 105 (48.2%) patients in Group A+B. The median (IQR) time to CRS onset was 2 days (2-3) across groups. ICANs events were identified in 6 (3.1%) patients in Group A (Group A+B: 2.8%), with median (IQR) onset time of 2.5 days (2-4). Almost all patients received dexamethasone during the SUD period (Group A: 192 [99.5%], Group A+B: 217 [99.5%]) and acetaminophen (Group A: 191 [99.0%], Group A+B: 216 [99.1%]). Other supportive medications received include diphenhydramine (Group A: 109 [56.5%], Group A+B: 124 [56.9%]) and tocilizumab (Group A: 101 [52.3%], Group A+B: 120 [55.0%]). Conclusions Nearly all adopters of elranatamab in the MDV database had complete SUD, with the majority receiving timely administration aligned with the label. Both the frequency and timing of CRS and ICANS were generally consistent with past findings from elranatamab clinical trials. Despite the occurrence of CRS and ICANS events, many patients continued to receive complete and timely SUD.