Background:

It has long been recognised that while pre-marketing clinical trials are effective in studying the efficacy of medicines there are a number of limitations in their ability to adequately define the safety profile of drugs. The power to detect rare or delayed adverse drug reactions (ADRs) in pre-marketing clinical trials is limited by the relatively small number of patients involved or the short duration of therapy in many trials. In addition, these studies often exclude patients as a result of their age , sex or concurrent diseases which can eliminate patients who may be more susceptible to ADRs. This therefore limits the generalisability of the results of such trials.


Post-marketing surveillance methods, on the other hand, are able to present a more realistic drug safety profile since they offer the potential to monitor ADRs in larger numbers of patients (>10,000), a greater variety of patients and for longer durations.


One of the most well known methods for post-marketing surveillance are spontaneous reporting systems such as the Yellow Card Scheme in the UK. These systems vary from country to country but generally they rely on voluntary reporting of ADRs to a centralised database. They have the advantage of covering very large numbers of patients and all drugs for their entire life cycle. Reporting is by healthcare professionals, the drug manufacturer or in some cases, patients. These systems are limited by under-reporting of ADRs, variable quality of reporting and since there is no denominator information for drug exposure, there are also limitations in their ability to provide information on the incidence of ADRs.


Furthermore, the reports in these systems are dependent on an initial suspicion that the drug may be causing an adverse reaction.