Background: Adherence is a prerequisite for the efficacy of any drug, and previous studies have shown that non-adherence is associated with disease activity and increased health care cost in multiple sclerosis (MS). The aim of this study was to investigate rates and reasons for discontinuation of dimethyl fumarate (DMF) among people with MS on a national level and differences between clinics in Denmark. Methods: This was a nationwide, registry and population study of patients treated with DMF. We calculated standard residuals (SR) demonstrate differences between clinics. For survival analysis regarding discontinuation rates and discontinuation due to specific AEs we used log-rank test Cox-proportional hazards and plotted Kaplan-Meier graphics. Results: We included 2,448 people with MS, treated with DMF from 2013 to 2020. Average treatment duration was 26 months (5,382 treatment years). 49.2 % of patients who initiated treatment with DMF (n = 1205) were continuously treated. Reasons for discontinuation were adverse events (54.5 %, n = 656), active disease (26.1 %, n = 315), pregnancy (9.4 %, n = 113) or other reasons (13.2 %, n = 159). We compared SR to the mean regarding reasons for discontinuation and found significant differences between sites regarding gastrointestinal adverse events, flushing and lymphopenia. Discontinuation due to all adverse events, flushing and lymphopenia were more frequent in female than male patients. Conclusion: In this population-based study, we found major differences between the MS clinics in rates and reason for discontinuation of DMF. Our results suggest that management strategies during DMF treatment can reduce discontinuation rates.