The editors of this volume have looked upon pharmacovigilance as being the study of the safety of marketed drugs under the practical conditions of clinical usage in large communities. However, some aspects of this definition need qualification: safety cannot be considered apart from efficacy in most situations. For example, an ineffective drug used in a serious and life-threatening disease would be unsafe. Those individuals conducting pharma-covigilance are concerned not only with marketed drugs but also with their pre-marketing data—but our working definition serves a practical purpose.

Spontaneous reporting of suspected adverse drug effects is central to pharmacovigilance— which is the systematic search for signals of drug toxicity. When such a signal is detected it has to be verified, explored, and understood—realising that the drug may be acceptably safe if used by individuals who are not at especially high risk by virtue of genetic constitution, metabolism, or other characteristics that could alter individual risk.

Pharmacovigilance is conducted by a very large number of people concerned with protecting populations from serious unintended adverse consequences of medication exposure. It is important to recognise that most medications carry some risk due to their pharmacologic properties or to other factors. The evaluation of risk must be conducted in the context of the patient benefit derived from treatment, the severity of the condition being treated, and other objective and subjective factors (such as the patient's values). Each of the stakeholders —the patient, physician, pharmaceutical company, academic investigator, government—may have a different perspective on the same set of evidence. For example, a patient may be willing to accept a high risk of side-effects for benefits of the treatment for a condition that might be considered trivial by others. A regulatory agency may consider the burden of the same side-effects to be too high, given their view of the risk-benefit equation. A governmental or third-party payer might see the issue from an even different perspective, since a payer may not wish to bear the cost of the treatment or the cost of treating an adverse event. It is perhaps not surprising that each group may take a different view of the same evidence. In addition, each group may also be swayed by intense external pressures to take action to protect specific interests, for example to protect the public against potential harm or to protect against legal liability. These pressures may lead to early decisions based on incomplete scientific data.

There have been mistakes and errors in the field of pharmacovigilance: some drugs have been withdrawn when the benefit to large numbers of patients has not been properly balanced against the harm done to very few highly susceptible subjects. Identifying the patients most susceptible to risk and finding ways to channel medications to the appropriate patients would have been more rational. It is always highly desirable to subject the signal to the formal processes of pharmacoepide-miology (such as case-control, cohort, large simple randomised trial, etc.) before taking gross action on a weak or questionable signal. We have to weigh benefit against risk and the benefit may be to a large population affected by a serious disease and the risk may be to a small population of susceptibles.

This book intends to help bring more rigorous considerations of scientific evidence to the various sectors that face critical decisions about how to act in the face of incomplete information. Our hope is that future decisions will be improved, and that public policy decisions can be made more transparent in the process.

The tension between regulator (government oversight agencies) and regulated (pharmaceutical industry) that was apparent in earlier years must be viewed in a more complex environment in which additional sectors also have considered opinions of the evidence, and possess strong interests as well. All sectors must grapple with the evidence and the pros and cons of decisions and the consequences of these decisions. The subject is not easy and its participants are frequently highly exposed. If this book is of any help to those exposed to political pressure, media pressure, their own indecision and anxieties, etc., then it will have been worth the effort taken to produce it.

The book falls into four parts: the basis of pharmacovigilance, signal generation, pharmacov-igilance and the system-organ classes, and, finally, lessons and directions. We have eliminated some duplication but not all of it: people come at the same thing from different directions and some times those different viewpoints need to be preserved. Some subjects, for a variety of reasons, have been inadequately covered (signal generation in important countries and developing areas of the world; dictionaries and MedDRA; pharmacovigi-lance as conducted by some of the big companies, other than those already reviewed; non-US medical devices legislation; renal, cardiovascular and respiratory adverse drug reactions, etc.). Some of these subjects have been left for expansion in our second edition; some have been omitted this time round because, for example, we do not intend to cover all the system-organ classes in each edition but will choose different themes as time goes by.

A large number of people are concerned with pharmacovigilance but they are a very small number compared with the populations that they set out to protect. We hope that this volume will be of help to them and we thank our many authors for their contributions.

The editors wish to express their considerable appreciation to John and Celia Hall who took over the management of the production of this book in difficult circumstances and whose contribution is much appreciated. Professor Mann also wishes to acknowledge the considerable support of his personal assistant, Mrs Susan Jerome.

Ronald D. Mann Elizabeth B. Andrews

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