Background Clinical trial populations do not fully reflect routine practice. The power of routinely collected data is increasingly recognised.

Methods The OPTIMISE:MS pharmacovigilance study is a prospective, pragmatic observational study, conducted across 13 UK MS centres. Data were collected at the time of routine clinical visits. The first participant was recruited on 24th May 2019; data were extracted on 11th November 2021.

Results 2112 participants were included (median age 44.0 years; 1570 (72%) female; 1981 (94%) relapsing- remitting MS). 639 (30%) were untreated at study entry, 205 (10%) taking interferon beta/copaxone, 1004 (47%) were taking second/third generation DMT first line (including dimethyl fumarate, oral cladribine, natalizumab, ocrelizumab), and 264 (13%) had escalated from either interferon beta or copaxone. 342 clinical events were reported, of which 108 were infections. There was an increased risk of adverse events in people taking second/third generation DMT (RR 3.45, 95%CI 1.57-7.60, p<0.01 vs no DMT). Unadjusted Poisson regression demonstrated increased incident adverse events in people taking natalizumab (IRR 5.28, 95%CI 1.41-19.74, p<0.05), ocrelizumab (IRR 3.24, 95%CI 1.22-8.62, p<0.05), and GA biosimilar (Brabio) (IRR 4.89, 95%CI 1.31-18.21, p<0.05) vs no DMT.

Conclusions Routinely collected healthcare data can be used to evaluate DMT safety in people with MS. These data highlight the potential of pragmatic studies to guide understanding of risks and benefits associated with DMT.