This is the first phase of clinical trials. The drug is introduced into human subjects after years of preclinical testing. The drug is administered at a variety of doses to achieve the therapeutic index of the drug. By identifying the toxic dose and the minimum therapeutic dose, one will determine some side effects and the metabolism of the drug into the human system. At this stage of trials, the number of subjects is fairly small and most of the subjects are fairly healthy.
In this phase, the goal is to determine the common short-term side effects and risks. The drug is administered at various doses to determine the most effective and best tolerated dose. The subjects typically have the disease or ailment that the drug was targeted for. The sample size is larger than the size for Phase I trials.
At this stage, usually the therapeutic dose has been determined and this is for further identification of side effects or other ailments caused by this compound. The goal is to gather additional information on the effectiveness/safety of the drug. The marketing and labeling of the drug will also be determined at this stage of development. Usually more subjects are admitted into this stage of development. If there are no major serious adverse effects that are attributed to the compound of study, the sponsor will usually apply for FDA approval after or during these Phase III trials.
Currently some aspects can be incorporated into Phase III trials. Phase IV studies are usually completed to determine cost-effectiveness for marketing the drug and the various doses the drug can be administered to the subjects. Most Phase IV trials are done when the sponsor determines the best way to market the product.