Pharmacotherapy in oncology is complex. It has a small therapeutic range, and is susceptible to errors. Medication errors – especially overdosage – often have serious effects while underdosage entails unnoticed impairment of the therapeutic success. Typical medications and procedures susceptible to errors are described: unintentional intrathecal injections, the application of platinum compounds and anthracyclines, confusion of total and single dose, errors of calculation and preparation, or in operation of infusion pumps. Suitable prevention strategies are standardisation, separation of intrathecal and intravenous chemotherapy, and use of specialized software for planning and calculating chemotherapy, as well as non-punitive notification and recording systems for errors and near-errors in treatment.