The therapeutic effect of most immunosuppressive agents is unspecific and therefore often limited by an increased risk of infection by viral, bacterial or fungal organisms as well as by an increased incidence of malignant neoplasms. This short review includes the most commonly used immunosuppressants such as corticosteroids, azathioprine, methotrexate, cyclophosphamide and cyclosporine. The most common risks of long-term corticosteroid treatment are Cushing-like changes, decreased glucose tolerance and the usually benign steroid diabetes. Also clinically important is osteoporosis, since it can be prevented by physical training, calcium supplementation and treatment with vitamin D if necessary. Although there is still no proof of a significantly increased risk of peptic ulcer during steroid therapy, patients may develop gastrointestinal hemorrhage and even perforation without producing pain while being treated with corticosteroids. Mineralocorticoid effects, such as salt and water retention, are seen only with hydrocortisone and prednisone, whereas with synthetic steroids such as dexamethasone, sodium retention is absent despite their strong antiphlogistic activity. The most important side effect of the cytotoxic agents azathioprine, methotrexate and cyclophosphamide is marrow suppression. Due to the high turnover of neutrophils, patients most frequently suffer neutropenia rather than thrombocytopenia or anemia. Neutropenia, as well as impaired humoral and cellular immune mechanisms, are responsible for increased susceptibility to bacterial, viral or parasitic diseases during immunosuppressive therapy. Hepatotoxicity has been reported among patients receiving azathioprine (cholestatic hepatitis) and methotrexate (elevated AST levels and, rarely, liver fibrosis or cirrhosis). Cyclophosphamide causes hemorrhagic cystitis in a substantial proportion of patients, as well as an increased incidence of urothelial neoplasms. Both these side effects may be prevented by Mesna. The most important side effects of cyclosporine are acute and chronic nephrotoxicity usually associated with significantly elevated plasma levels of the drug. It must be borne in mind that severe nephrotoxicity may occur in patients receiving cyclosporine and ketoconazole together, since the latter may inappropriately increase the plasma cyclosporine level.