OBJECTIVE: To assess whether treatment with the generic drug domperidone can reduce the progression of disability in secondary progressive multiple sclerosis (SPMS), we conducted a phase 2 futility trial following the Simon 2-stage design. METHODS: We enrolled patients in an open-label, Simon 2-stage, single-center, phase 2, single-arm futility trial at the Calgary Multiple Sclerosis Clinic if they met the following criteria: age of 18 to 60 years, SPMS, screening Expanded Disability Status Scale score of 4.0 to 6.5, and screening timed 25-ft walk (T25FW) of ≥9 seconds. Patients received domperidone 10 mg 4 times daily for 1 year. The primary outcome was worsening of disability, defined as worsening of the T25FW performance by ≥20% at 12 months compared to baseline. This trial is registered with ClinicalTrials.gov (NCT02308137). RESULTS: Between February 13, 2015, and January 3, 2020, 110 patients were screened, 81 received treatment, and 64 completed follow-up, of whom 62 were analyzed. The study did not meet its primary endpoint: 22 of 62 (35%) patients experienced significant worsening of disability, which is close to the expected proportion of 40% and above the predefined futility threshold. Patients with higher prolactin levels during the study had a significantly lower risk of disability progression, which may warrant further investigation. Domperidone treatment was reasonably well tolerated, but adverse events occurred in 84% and serious adverse events in 15% of patients. CONCLUSIONS: Domperidone treatment could not reject futility in reducing disability progression in SPMS. The Simon 2-stage trial model may be a useful model for phase 2 studies in progressive MS. TRIAL REGISTRATION INFORMATION: ClinicalTrials.gov Identifier: NCT02308137. CLASSIFICATION OF EVIDENCE: This study provides Class III evidence that in individuals with SPMS participating in a futility trial, domperidone treatment could not reject futility in reducing disability progression at 12 months.