BACKGROUND: Extended first-line therapy (1LT) has improved clinical outcomes in newly diagnosed multiple myeloma (NDMM). This retrospective study of NDMM patients evaluated the relationship between dose-attenuation of 1LT and duration of therapy (DOT) and DOT on outcomes. METHODS: Adults with NDMM not undergoing stem cell transplant (SCT) from January 1, 2012 toMarch 31, 2018 from the Integrated Oncology Network were included; 300 were randomly selected for chart review. 1LT DOT, time to next treatment (TTNT), progression-free survival (PFS), and overall survival (OS) were estimated using Kaplan-Meier analysis. Marginal structural models evaluated relationships between DOT and TTNT, PFS, and OS at 2 years accounting for confounders and survival bias from the time-dependent nature of DOT. RESULTS: Of 300 chart-reviewed patients, 93 were excluded for incomplete data or meeting exclusion criteria. Among 207 NDMM patients, median age was 74 years; 146 (70.5%) did not receive dose-attenuation during 1LT. Patients with short DOT were older, frailer, with a higher comorbidity burden, and a significantly lower proportion had an Eastern Cooperative Oncology Group PS = 0. As DOT increased, more patients underwent dose-attenuation (p < 0.0001). The median 1LT DOT was 20.9 (95% confidence interval [CI]: 13.9, 26.4) versus 4.2 months (95% CI: 3.2, 4.9) for patients receiving versus not receiving dose-attenuation, respectively (p < 0.0001). After accounting for survival bias, confounder-adjusted TTNT was prolonged with each additional month of 1LT (odds ratio [OR]: 0.76 [95% CI: 0.75, 0.78]); likelihoods of risks of disease progression (OR: 0.87 [95% CI: 0.86, 0.88]) and death at 2 years (OR: 0.72 [95% CI: 0.70, 0.74]) were reduced with each month of 1LT (p < 0.0001 for all outcomes). CONCLUSIONS: Dose-attenuated 1LT was associated with longer DOT among patients with non-SCT NDMM. Each additional month of 1LT was associated with a reduced adjusted likelihood of disease progression and death at 2 years. Dose-attenuation of 1LT can extend DOT; longer DOT may improve clinical outcomes.