EMA: Safety monitoring of medicines: overview of the Agency’s activities

Before a medicine is authorised for use, any experience of its safety and efficacy is limited to its use in clinical trials. Therefore, at the time of a medicine’s authorisation, it will only have been tested in a relatively small number of patients for a limited length of time as part of a clinical trial. As a result some adverse reactions may not be seen until a very large number of people have received the medicine and begin to use it over longer time periods.