BACKGROUND:Differential treatment allocation may impact on clinical phenotype in MS and in turn upon quality of life (QoL). OBJECTIVES:(a) Investigate the association between disease-modifying drugs (DMDs) use and relapse frequency, disability, clinically significant fatigue, and physical and mental health-related QoL among participants with MS residing in Australia and New Zealand (NZ); (b) assess whether these associations differed between Australia and NZ. METHODS:Disability and fatigue were measured by PDDS and FSS, respectively. QoL was assessed by MSQOL-54. Associations were assessed by binomial and multinomial logistic regression, as appropriate. Multivariable models were adjusted for demographic and clinical covariates, as appropriate. RESULTS:837 participants (627 from Australia; 210 from NZ) were identified from an online cohort of people with MS. First- and second-generation DMD use was associated with higher adjusted-odds of fatigue and disability, though not with 12-month relapse number. DMD use was not independently associated with physical or mental QoL. The association of first-generation DMD use with moderate disability differed between nations, such that treatment was associated with lower odds in Australia but not in NZ; a similar but a small difference was found for severe disability. No differences were seen in the DMD association with relapse number, nor with fatigue or QoL, between Australia and NZ. CONCLUSION:The differential treatment allocation associations in NZ are evident in the DMD-disability association, but there is no evidence that this treatment regimen has negative associations with fatigue, mood, or QoL.