Why there is a need for pharmacovigilance.

Not everything is known about a medicine when it receives its licence for marketing. The merits of a new drug, balancing its beneficial and its untoward effects, become only established after sufficient experience has been gained from its use in real practice. Part of the reason for this is that our extensive phase III clinical trials fail to detect some side-effects. Why is this so? Three groups of reasons may be envisaged, namely (1) our trials lack the power to detect rare side-effects; (2) some side-effects do not occur in the context of clinical trials; (3) some side-effects, though common enough, fully or partly escape detection due to lack of suitable detection techniques. The following presents a closer look at these three mechanisms. Copyright 1999 John Wiley & Sons, Ltd.