Prescribing a drug represents a trade-off between its benefits and harms. The harm to benefit ratio varies according to the condition being treated, the drug's pharmacology, and the availability and safety of other therapeutic options. A decision to prescribe a drug therefore has to be made on the basis of all the information available. There is, however, often an imbalance in such information. Although high quality data regarding benefits usually come from randomised controlled trials, systematic reviews, and evidence synopses, information on harms may be limited. This is particularly the case for new drugs, which usually have been given to only a few thousand patients, and often for relatively short periods before licensing.1 This allows common adverse events to be detected, but not rare events or those that occur only with prolonged exposure. Consequently, established compounds continue to cause much harm2—for example, approximately 5% of all admissions to US and European hospitals are due to adverse drug reactions.^w1 A much publicised meta-analysis showed that such reactions caused more than 100 000 deaths in the United States in 1994.^w2

Monitoring harms after launch of a product is thus crucial, since regulatory action may be necessary to protect public health. Systems to monitor drug safety are well developed in the …