<P> The phases of clinical research are the steps in which scientists do experiments with a health intervention in an attempt to find enough evidence for a process which would be useful as a medical treatment . In the case of pharmaceutical study, the phases start with drug design and drug discovery then proceed on to animal testing . If this is successful, they begin the clinical phase of development by testing for safety in a few human subjects and expand to test in many study participants to determine if the treatment is effective . </P> <P> Clinical trials involving new drugs are commonly classified into four phases . Individual trials may encompass more than one phase . A common example of this is combined phase I / II or phase II / III trials . Therefore, it may be easier to think of early phase studies and late phase studies . The drug - development process will normally proceed through all four phases over many years . If the drug successfully passes through Phases I, II, and III, it will usually be approved by the national regulatory authority for use in the general population . Phase IV are' post-approval' studies . </P> <Table> <Tr> <Th_colspan="7"> Summary of clinical trial phases </Th> </Tr> <Tr> <Th> Phase </Th> <Th> Primary goal </Th> <Th> Dose </Th> <Th> Patient monitor </Th> <Th> Typical number of participants </Th> <Th> Success rate </Th> <Th> Notes </Th> </Tr> <Tr> <Td> Preclinical </Td> <Td> Testing of drug in non-human subjects, to gather efficacy, toxicity and pharmacokinetic information </Td> <Td> unrestricted </Td> <Td> scientific researcher </Td> <Td> not applicable (in vitro and in vivo only) </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Phase 0 </Td> <Td> Pharmacokinetics; particularly, oral bioavailability and half - life of the drug </Td> <Td> very small, subtherapeutic </Td> <Td> clinical researcher </Td> <Td> 10 people </Td> <Td> </Td> <Td> often skipped for phase I </Td> </Tr> <Tr> <Td> Phase I </Td> <Td> Testing of drug on healthy volunteers for safety; involves testing multiple doses dose - ranging </Td> <Td> often subtherapeutic, but with ascending doses </Td> <Td> clinical researcher </Td> <Td> 20--100 normal healthy volunteers (or for cancer drugs, cancer patients) </Td> <Td> approximately 70% </Td> <Td> determines whether drug is safe to check for efficacy </Td> </Tr> <Tr> <Td> Phase II </Td> <Td> Testing of drug on patients to assess efficacy and side effects </Td> <Td> therapeutic dose </Td> <Td> clinical researcher </Td> <Td> 100--300 patients with specific diseases </Td> <Td> approximately 33% </Td> <Td> determines whether drug can have any efficacy; at this point, the drug is not presumed to have any therapeutic effect whatsoever </Td> </Tr> <Tr> <Td> Phase III </Td> <Td> Testing of drug on patients to assess efficacy, effectiveness and safety </Td> <Td> therapeutic dose </Td> <Td> clinical researcher and personal physician </Td> <Td> 300--3,000 patients with specific diseases </Td> <Td> 25--30% </Td> <Td> determines a drug's therapeutic effect; at this point, the drug is presumed to have some effect </Td> </Tr> <Tr> <Td> Phase IV </Td> <Td> Postmarketing surveillance--watching drug use in public </Td> <Td> therapeutic dose </Td> <Td> personal physician </Td> <Td> anyone seeking treatment from their physician </Td> <Td> N / A </Td> <Td> watch drug's long - term effects </Td> </Tr> </Table> <Tr> <Th_colspan="7"> Summary of clinical trial phases </Th> </Tr>

When is a drug studied in patients who have the condition that the drug is intended to treat