Background <p>Subarachnoid hemorrhage (SAH) is a severe neurological condition with high rates of disability and mortality. Although nimodipine is widely used in the treatment of SAH, the potential benefits of magnesium as an adjunct therapy remain unclear. The aim of this study was to explore the impact of magnesium sulfate combined with nimodipine on mortality in patients with non-traumatic SAH (NSAH).</p> Methods <p>This retrospective cohort study was based on the Medical Information Mart for Intensive Care IV (MIMIC-IV) database, including critically ill patients with NSAH. Patients were categorized into a combined group (N + M) and a nimodipine-only group (N) based on their use of magnesium sulfate and nimodipine during their ICU stay. The primary outcome was one-month all-cause mortality, while the secondary outcome was one-year all-cause mortality. Multivariable analysis was used to adjust for confounding factors. Landmark analysis was performed to assess both short-term and long-term effects.</p> Results <p>A total of 587 patients were included in the study, with 280 in the N + M group. The one-month and one-year all-cause mortality rate were 15% and 20%, respectively, for the N + M group, compared to 7.2% and 9.1% for the N group. The use of magnesium sulfate was associated with higher one-month (HR 1.89 [95% CI 1.09–3.27]) and one-year (HR 2.08 [95% CI 1.29–3.36]) mortality. Landmark analysis showed that the mortality risk between the two groups remained consistent from two months to one year.</p> Conclusion <p>In critically ill NSAH patients, the combination of magnesium sulfate and nimodipine was associated with increased all-cause mortality compared to nimodipine alone.</p> Clinical trial number <p>Not applicable.</p>