Testing of a new therapy or treatment trial progresses in an orderly manner which is known as phases. Which make researchers able to ask and answer questions in a way that results in reliable information about the drug and the patients? Clinical trials are usually classified into four phases.

This is the first and foremost stage of any clinical trial. In this stage the drug gets tested on human beings. For this phase only a small no of people, usually 20-80 , are taken. The people involved in the trial are the people who volunteer themselves for the study and are healthy. In this phase, safety, acceptability Absorption, Distribution, Metabolism and Excretion of a drug is administered. These trials are generally conducted in the clinics with the inpatient facility so that the subject or the volunteer who has been given the drug, could be under round the clock observation. In this phase usually only healthy volunteers are taken but in some special cases like cancer and HIV, drug trials are only done on the patients suffering from these dieseases.

As soon as the initial safety of the drug is ensured in phase I trials, Phase II trials are initiated. Phase II trials take a larger group on which the drug has to be administered. The group usually consists of 20-300 volunteers. Phase II mainly asseses the drug on the basis of its performance in the previous phase or in phase I. The development process failure for a new drug, usually takes place during Phase II when the drug is discovered to not work as planned, or have toxic effects.

These trials are initiated on comparitively a very large group of people or patient groups. Group size consists usually of 300-3000 patients but can increase as per the requirement. As opposed to Phase I trials, Phase III trials are performed in more then one centre or clinic. The major advantages of trials conducted in various centres include a larger number of participants, different geographic locations and the possibility of including of a wider range of population groups which enhances the ability to compare results among centers. The more varied results are obtained, the more beneficial they prove for the study.

This phase initiates after the drug is allowed to be sold in the market or is made available for the general public.These trials are carried out mainly by the sponsors of the drug to find a new market for the drug, or for many other reasons like can this drug be administered with any other drugs or how well it interacts with the different types of drugs.