Adverse effects of antibiotics can cause a failure of antibiotic treatment. The authors give a survey of antibiotic toxicity manifestations, according to the target organ systems, with emphasis on identification of at-risk patients and on possible prevention of particular adverse effects. Although antibiotics belong to relatively safe pharmaceuticals, many of them can be a cause of a serious damage to the human organism. Beta-lactam antibiotics are considered the least dangerous. A considerable number of adverse effects, especially the dose-dependent ones, are preventable on condition that the risk factors, as the patient's age, functional capacity of eliminating organs (kidney, liver), associated diseases and simultaneous administration of drugs, are considered. In conclusion, the clinically significant drug interactions of antibiotics are pointed out, being of increasing importance especially in patients with multiple diseases and polypragmatic manner of treatment. (Tab. 2, Ref. 48.)