Modern drugs are generally evaluated according to three major criteria: efficacy, safety, and cost-effectiveness. Studies to address these criteria begin once a compound is discovered. At any stage of drug development, the process can be terminated if the compound fails to meet these criteria. Even if a drug survives the pre-market testing and is introduced to the market, it can be withdrawn if adverse effects later prove to be unacceptable. Drug evaluation includes 4 phases that – in stepwise manner of number of patients, characteristics of patients and trial design, and complexity of patients and trial design – aim to provide the information for eventual product. With the introduction of more and more modern drugs and the dramatic increase in drug consumption and health care costs, more demand is being placed on the tools and techniques needed for generating data for decision makers at the various stages of drug evaluation. Pharmacoepidemiology, which specifically addresses this need, is an important discipline that has gained recognition and prominence in recent decades.

Pharmacoepidemiology is traditionally defined as the discipline concerned with the study of the use and effects of drugs in large numbers of people. It applies epidemiologic methods, knowledge, and reasoning to the subject of clinical pharmacology and therefore can be considered a subdiscipline of both clinical pharmacology and epidemiology. The epidemiologic methods used by this discipline range from single case reports to the observational or non-experimental population-based approach with several years of follow-up, to large-scale randomized clinical trials. Historically, the field of pharmacoepidemiology began with a focus on safety evaluation or the study of adverse drug reactions, particularly Type B reactions, which tend to be uncommon, dose-unrelated, unpredictable, and potentially more serious than Type A, i.e., dose-related and pharmacologic, reactions. It has evolved to include the study of the effectiveness of new drugs and the use of drugs post-marketing, such as patterns of and variations in prescribing in a particular health care facility or area, and strategies to improve the use of the drug. Recent extended applications that apply the population perspective to improve rational drug therapy have enhanced the impact of the field, and include studies of drug utilization, evaluating and improving physician prescribing, the development of treatment guidelines, drug utilization review, risk management, and the development of national drug policies. Another major area of drug evaluation, economic assessment, is discussed elsewhere in this book.

The field of pharmacoepidemiology has expanded enormously since the publication of the last edition of this book. Numerous research articles have been published and there are now many journals competing to accommodate those works. In addition, interest in further training in this discipline is rapidly increasing, as well as the number of training programs. The essence of the discipline has been incorporated into many postgraduate training programs in the medical sciences, such as clinical epidemiology, public health, clinical pharmacology, etc. Pharmacoepidemiology has contributed significantly to the area of regulatory approval and control, and it will continue to impact this area as long as drugs are permitted to enter the market with potentially unknown adverse side effects. The objective of this chapter is to summarize and describe important methods and applications in the field of pharmacoepidemiology, with a focus on developing countries.