Phases of clinical trials

Clinical trials can only be done if that particular drug is found to be safe in various preclinical studies. Preclinical studies are conducted on various animal species to test any carcinogenic, teratogenic or mutagenic potential of a particular drug. When a particular drug is found to be safe in preclinical studies then regulatory bodies give approval to conduct clinical trial.
Clinical trial consist of four phases. In phase I drug is tested on normal human volunteers to know about pharmacokinetics and pharmacodyanamics of that particular drug. In phase II drug is tested on both healthy volunteers as well as patient to know the safety and efficacy of that particular drug. In phase III drug is tested on patients to know about safety and efficacy of that drug but in phase III sample size is relatively larger as compared to phase II. In phase IV or post marketing surveillence phase any adverse effect is supervised on large sample of population to know about any rare side effect.