First of all, if you want to know what the Clinical Trial is then click on the link to find about Clinical Trials.
After discovery of the drug and preclinical testing, Clinical Trials/Research of any new drug passes following standard four phases:
Phase I - A small group of healthy volunteers(about 20 to 80) is selected in this very first phase of clinical trials. These people will be kept under observation for the evaluation of the drug safety, determination of safe dosage range and identify side effects.
Phase II - This second phase of clinical trials consists of larger group of volunteers(about 100 to 300) . This phase study is mainly focused on drug efficacy and how well the drug works. This phase further evaluates the experimentals drug's safety.
Phase III - This third phase of clinical trials consists of larger group of patients of different medical condition(about 1000 to 5000) . This phase's study is focused on assessment of the drug safety data to support marketing claims for the drug. In this phase the drug is used to monitor side effects and compare it with other commonly used treatments.
Phase IV - This fourth phase is also known as Post-marketing surveillance as these studies are done after the drug has been introdued to market. These studies are conducted over much larger portion of the patients to continue testing the drug to collect information about drug's effect in various populations. This study also gathers the data on any side effects which could result from long-term use of the drug or the side-effect which could be seen after a long term of drug usage.