Introduction: prior to present new drugs to market, it is essential to check them by several pharmacokinetic and pharmacodynamic tests, which are including 6 to 25 participants and the phase I to III clinical trails tests. These are consisted of 20-80,100-300 and 1000 to 3000 human participants respectively. Occasionally, we face with some drugs, which are being relapsed from market, even though they had passed whole the necessary tests. It seems that premarketing studies are not sufficient from statistical point of view to detect these cases. An adverse effect that occurs in only 1 in 2500 persons would go unnoticed when sample size is small. Once the drugs are marketed they often reach millions of patients and such events can become manifest. It is important to mention that the participants in the premarketing studies are healthier than whom they use medicine in post-marketing phase. pharmacoepidemiology can play an effective, role in such cases, by using its especial detecting methods. It is in this post marketing phase that pharmacoepidemiology come into play and with especial tools studies the various aspects of drugs.Methods: There are two types of pharmacoepidemiological studies-interventional and observational studies .Interventional studies categorize as randomized clinical trials, field trials and community trials. Observational studies are involved prospective cohort, retrospective cohort, Case-control, cross-sectional ecological studies, case series and case report.Conclusion: Pharmacoepidemiology with these especial tools provides especial facility in study of drugs.