A drug being studied that has not been approved for use by the Food and Drug Administration. Under certain circumstances, the use of investigational drugs is permitted for the treatment of serious or life threatening conditions for which no alternative treatment exists. Four requirements must be met before an investigational drug can be used to treat patients: the drug must be intended to treat a serious or life threatening disease, there cannot be a satisfactory alternative therapy available, the drug must already be under investigation or clinical trials must be completed, and the trial sponsor must be seeking approval to market the drug.