Clinical trials involving new drugs are commonly classified into five phases. Each phase of the drug approval process is treated as a separate clinical trial. The drug-development process will normally proceed through all four phases over many years. If the drug successfully passes through phases 1, 2, and 3, it will usually be approved by the national regulatory authority for use in the general population. Before pharmaceutical companies start clinical trials on a drug, they will also have conducted extensive preclinical studies . Each phase has a different purpose and helps scientists answer a different question.