Aim
Diabetes mellitus (DM) and multiple sclerosis (MS) are two common diseases known to worsen the trajectory of each other, yet it is unknown whether MS is associated with incident DM.

Methods
Using Danish nationwide registries, we identified all patients aged 18–99 with a first-time primary or secondary discharge diagnosis with MS between 2000 and 2018, with no known DM. These patients were matched with control subjects from the background population in a 1:5 ratio based on age and sex, to assess their risk of DM.

Results
A total of 13,376 patients with MS and 66,880 matched control subjects were included (33 % men; median age, 42 years [25th–75th percentile, 33–51]). During a median follow-up of 8.3 years (25th–75th percentile, 4.0–13.3), 467 (3.5 %) patients with MS and 2397 (3.6 %) control subjects were diagnosed with DM. The cumulative incidence of DM was similar among patients with MS and control subjects (95 % confidence interval [CI] 6.5 % [5.7–7.2 %] vs. 7.3 % [95 % CI 6.9–7.9 %], respectively), and adjusted analysis yielded a hazard ratio (HR) of 0.98 [95 % CI 0.89–1.09]). The overall risk of incident type 1 diabetes was low and yielded a HR of 1.60 [95 % CI 0.98–1.40] in patients with MS compared with control subject (P = 0.07).

Conclusion
This study demonstrated that patients with MS had a similar risk of incident DM as compared to age- and sex matched controls from the background population.