The granting of regulatory approval allows medical practitioners to prescribe a drug in a controlled way to a group of patients defined within the licence. Prior to this, the new product may have been evaluated often in less than 5000 patients and usually in a selected environment in which many patients have been excluded, including for example, women of childbearing potential, the elderly and children. Co-existent disease and the concomitant use of a number of common drug treatments also frequently exclude patients from pre-licensing trials. It is hardly surprising, therefore, that many adverse drug reactions are only detected once the product has been prescribed to the general population. National and international regulatory bodies, therefore, provide systems for post-marketing pharmacosurveillance, although participation in these by clinicians is generally voluntary and under-reporting is widespread. Post-marketing surveillance (PMS) studies are not generally an integral component to launching a new drug and many clinicians are sceptical over data generated in trials which do not conform to the 'gold standard' randomised control trial (RCT) design. However, in dismissing such studies, a great opportunity to obtain information, often from many thousands of subjects, is being missed. This article discusses post-marketing pharmacovigilance and the role of PMS studies in the context of current UK and European legislation.