Phase I - generally involves a small number of patients and its primary goal is to determine if the treatment is safe for use in humans.
Phase II – larger than a phase I and used to determine the efficacy of the treatment, looking at whether the treatment provided benefitted most participants. If an acceptable percentage of the patients respond well to the drug, it will go to a phase III trial.
Phase III – this is largest and usually the longest stage of the process. In stage 3, the drug or intervention is compared to the current standard of care for that disease state to determine if it works better. Several hundred patients are involved from many different regions or countries and are monitored for their response to the drug as well as any potential side effects.
Phase IV: conducted after the trial drug has already hit the market. This phase is gathering additional information on the long-term effects of a much broader patient population.