Abstract: Objective: To determine whether oral vitamin D supplementation reduces the risk of glucocorticoid (GC)-associated severe adverse events (SAEs) in patients with giant cell arteritis (GCA) included in the Spanish ARTESER registry. Methods: The ARTESER registry collected data from patients diagnosed with GCA across 26 Spanish public hospitals between June 2013 and March 2019. SAEs were defined as fatal, life-threatening, or requiring hospitalization. Patients were categorized according to the use or non-use of oral vitamin D supplements. Incidence rates (IRs) of SAEs were expressed per person-year with 95% confidence intervals (CIs). Cox proportional hazards models assessed vitamin D supplementation and its interaction with cumulative glucocorticoid dose. Results: Of 1568 patients (mean age 76.9 ± 8.1 years; 70.1% women) receiving GC, 120 (7.6%) experienced SAEs (IR 0.039; 95% CI 0.033-0.047). Vitamin D supplementation was documented in 1186 (75.6%) compared with 382 (24.4%) non-supplemented patients. SAE incidence was similar in supplemented (n = 89; 7.5%; IR 0.038, 95% CI 0.030-0.046) and non-supplemented patients (n = 31; 8.1%; IR 0.045, 95% CI 0.031-0.064) (p = 0.387). Multivariable Cox regression showed a significant interaction between vitamin D supplementation and cumulative glucocorticoid dose (interaction term HR 0.90; p = 0.033), consistent with a dose-dependent protective effect. Conclusions: Vitamin D supplementation was not independently associated with a lower incidence of GC-related SAEs, likely due to residual confounding factors. However, the interaction with cumulative GC exposure suggests a modulatory effect. Prospective studies incorporating stratified baseline vitamin D assessments are warranted.

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