Pharmacoepidemiology is the study of the uses and effects of drugs in well-defined populations.

To accomplish this study, pharmacoepidemiology borrows from both pharmacology and epidemiology. Thus, pharmacoepidemiology is the bridge between both pharmacology and epidemiology. Pharmacology is the study of the effect of drugs and clinical pharmacology is the study of effect of drugs on clinical humans. Part of the task of clinical pharmacology is to provide a risk benefit assessment by effects of drugs in patients:

doing the studies needed to provide an estimate of the probability of beneficial effects on populations, or assessing the probability of adverse effects on populations.

Other parameters relating to drug use may benefit epidemiological methodology. Pharmacoepidemiology then can also be defined as the transparent application of epidemiological methods through pharmacological treatment of conditions to better understand the conditions to be treated.