By the time medicinal products are granted a marketing authorisation, knowledge about their safety is incomplete because the clinical trials investigating the safety profile and efficacy are usually performed with a relatively small number of subjects. Furthermore these subjects are selected with regard to special aspects and in general do not represent the average patient in medical practices. Therefore, rare or very rare adverse drug reactions, interactions or other hazards related to the use of a medicinal product cannot be observed during clinical trials. Especially the rarely occurring serious adverse reactions are very important for the overall assessment of a new medicinal product. New findings regarding the safety of a medicinal product may occur even long time after its authorisation and depend on new developments in medical science.