Phase III

Following Phase II testing, a drug is tested in a large-scale setting (several hundred to several thousand patients) to determine effectiveness, benefits, and the range of possible adverse reactions. Most Phase III studies are randomized and double blinded, and typically last several years. Usually, two well-controlled studies are necessary to establish efficacy. Once Phase III trials are successfully completed, a pharmaceutical company can request FDA approval for marketing the drug, by filing a New Drug Application (see NDA).