BACKGROUND: Corticosteroids appears to be beneficial for severe Mycoplasma pneumoniae pneumonia in children, but data in adults are limited. This study investigated effects of adjunctive corticosteroids in hypoxemic adults with M. pneumoniae pneumonia. METHODS: Adults admitted 2013-2017 with verified M. pneumoniae pneumonia and hypoxemia (SpO2 <
93% or oxygen treatment) were included in a cohort. Treatment was defined as receipt of at least 1 glucocorticoid dose.Primary outcome was time to regression of hypoxemia, analyzed with a multivariable Cox regression. Secondary outcomes included fever duration, length of stay, and complications. RESULTS: Corticosteroids were given to 31% (122/388) during hypoxemia. Median age was 44 (interquartile range [IQR] 34-57) years. Median time to start of corticosteroid treatment was 1.9 (IQR 0.6-3.6) days from admission. Median cumulative dose was equivalent to 15 (IQR 10-19) mg betamethasone. Treatment duration was 5 (IQR 3-6) days. Patients treated with corticosteroids had more severe respiratory disease, longer symptom duration, and were more often treated with fluoroquinolones.Time to regression of hypoxemia (hazard ratio [HR] 0.92 [95% confidence interval {CI}: .72-1.19], P = .53) and length of stay (HR 0.91 [95% CI: .71-1.16], P = .44) were not significantly different between corticosteroid treated and controls. Corticosteroid treatment was associated to shorter fever duration (HR 1.44 [95% CI: 1.00-2.06], P = .046). Complications did not differ significantly between treatment groups. CONCLUSIONS: Adjunctive corticosteroids were not associated with reduced time to regression of hypoxemia in adults with M. pneumoniae pneumonia. However, duration of fever was shorter and no increase in complications was seen.