This exploratory study analyzed the medication preparation and administration process in medical-clinical units at four Brazilian hospitals located in the Southeast, Central-West and Northeast, identified problems that can contribute to the occurrence of medication errors and proposed improvement measures. Data were collected through non-participant and direct observation of nursing professionals' activities during one week. The results revealed the following main problems: the environment in Hospital B; preparation errors related to technique and anticipated medication preparation in C and D; technical, communication and patient identification mistakes in Hospital A. Suggestions to achieve greater quality and safety in patient care include the constitution of a multidisciplinary commission to evaluate the system, nursing discussion groups, environmental improvements, permanent courses and training and actual presence of a nurse during the process.