Pharmacies permeate and interconnect various actions developed in different sectors within the complex process of the use of drugs in a hospital. Dispensing failures mean that a breach has occurred in one of the last safety links in the use of drugs. Although most failures do not harm patients, their existence suggests fragility in the process and indicates an increased risk of severe accidents. Present concepts on drug-related incidents may be classified as side effects, adverse effects, and medication errors. Among these are dispensing errors, usually associated with poor safety and inefficient dispensing systems. Factors associated with dispensing errors may be communication failures, problems related to package labels, work overload, the physical structure of the working environment, distraction and interruption, the use of incorrect and outdated information sources and the lack of patient knowledge and education about the drugs they use. So called banal dispensing errors reach significant epidemiological levels. The purpose of this paper, which is part of a study on the occurrence of dispensing errors in the pharmacy of a large hospital, is to review the main concepts that guide studies on adverse effects and to provide an update on dispensing errors.