Side effects of long-term oral corticosteroid therapy. Systemic corticosteroid therapy has been used for over 70 years, and is still the cornerstone of the treatment of many conditions, in particular systemic, autoimmune or inflammatory diseases. Side effects of corticosteroids are numerous, and for most of them well known by prescribers. Nonetheless, guidelines for the prevention of corticosteroids toxicity are scarce, and the implementation of protective measures by prescribers is heterogenous. Hence, corticosteroids related complications entail a significant morbidity, which, importantly, could be largely prevented. We conducted therefore a systematic literature, through the Medline database, the Cochrane database and the grey literature until January 2021. After recalling the history of the discovery of corticosteroid therapy and its main pharmacological properties, we present the various complications associated with long-term corticosteroid therapy, and discuss the relevance of the preventive measures that may be proposed in daily practice in the light of available scientific evidence. This work highlights the importance of multidisciplinary follow-up, but above all of a thorough screening of the risk factors of complications at treatment initiation, and of a repeated evaluation of the complications all along the treatment course, in order to reduce the significant burden of morbidity associated with long-term corticosteroid therapy and to improve patient quality of life.