Background: Oral corticosteroids (OCS) are often prescribed for chronic obstructive pulmonary disease (COPD) exacerbations. Methods: This observational, individually matched historical cohort study used electronic medical records from the UK Clinical Practice Research Datalink linked to English Hospital Episode Statistics (HES) to evaluate adverse outcomes in patients with COPD who used OCS (OCS cohort) and those not exposed to OCS (non-OCS cohort). Risk of 17 adverse outcomes was estimated using proportional hazard regression. Results: Of 323,722 patients, 106,775 (33.0%) had COPD-related OCS prescriptions. Of the 106,775 patients in the overall cohort, 58,955 had HES linkage and were eligible for inclusion in the OCS cohort. The individual matching process identified 53,299 pairs of patients to form the OCS and non-OCS cohorts. Median follow-up post-index was 6.9 years (OCS cohort) and 5.4 years (non-OCS cohort). Adjusted risk of multiple adverse outcomes was higher for the OCS cohort versus the non-OCS cohort, including osteoporosis with/without fractures (adjusted hazard ratio [aHR] 1.80; 95% confidence interval [CI] 1.70-1.92), type 2 diabetes mellitus (aHR 1.44; 95% CI 1.37-1.51), cardiovascular/cerebrovascular disease (aHR 1.26; 95% CI 1.21-1.30), and all-cause mortality (aHR 1.04; 95% CI 1.02-1.07). In the OCS cohort, risk of most adverse outcomes increased with increasing categorized cumulative OCS dose. For example, risk of cardiovascular/cerebrovascular disease was 34% higher in the 1.0-<2.5 g group versus the <0.5 g group (HR 1.34; 95% CI 1.26-1.42). Conclusion: Any OCS use was associated with higher risk of adverse outcomes in patients with COPD, with risk generally increasing with greater cumulative OCS dose.Plain Language Summary: Many patients with chronic obstructive pulmonary disease (COPD) have occasions when their symptoms suddenly worsen, called flare-ups or exacerbations. To treat flare-ups, doctors might prescribe a course of steroid tablets (oral corticosteroids or OCS for short). Doctors might also prescribe "rescue packs" containing OCS and antibiotics, to keep at home and start taking when needed.While OCS may speed up recovery from flare-ups, repeated use may have negative health effects. We studied effects of OCS use in patients with COPD, using anonymized electronic patient medical records in England. These databases are made available following a high-quality research proposal to research and ethics committees.Of 323,722 patients with COPD, around one-third received OCS for flare-ups. We studied 17 outcomes including important medical diagnoses and death. We grouped patients into 53,299 pairs so that every patient who used OCS matched a similar patient (eg, the same age and sex) who never used OCS. The patients were followed for an average of 6.9 years (used OCS) and 5.4 years (never used OCS).Most diagnoses, including diabetes, osteoporosis, cardiovascular/cerebrovascular disease, and death, were more likely in patients who used OCS than those who never used OCS. Patients using la...