Impact of cancer on multiple sclerosis–related healthcare and disease-modifying drug use: A multinational cohort study

BACKGROUND: Neurological care and disease-modifying therapies (DMTs) are central to multiple sclerosis (MS) management, but how comorbidities like cancer affect them remains unclear. OBJECTIVE: We assessed the impact of cancer on MS-related healthcare and DMT use. METHODS: This cohort study accessed population-based data from France (2009-2021) and British Columbia, Canada (1991-2020). Cases were individuals with MS diagnosed with an incident cancer. The 4-year study period encompassed the 2 years before and after the cancer diagnosis. Each case was matched to two cancer-free individuals with MS (i.e. controls). Outcomes were neurologist visit rates, MS hospitalization rates, and percentage of DMT users. Mixed-effects models were used with a knot at cancer diagnosis. Incidence rate ratios (IRRs) were estimated pre- and post-cancer (cases vs. controls) and compared using a slope ratio (SR). RESULTS: In total, 6902 cases were matched to 13,804 controls. There was a negative effect of cancer on DMT use (SR = 0.36 [0.19-0.70]), especially among those on chemotherapy (SR = 0.24 [0.14-0.40]). Neurologist visit IRRs did not differ pre- and post-cancer (SR = 0.96 [0.91-1.005]). Similarly, MS hospitalization IRRs did not differ (SR = 1.06 [0.84-1.33]). CONCLUSION: Following cancer, DMT use declined sharply. However, there was no evidence of a strong impact of cancer on neurologist visits and MS hospitalizations.