Drug development is a highly complex process that involves multiple steps of preclinical and clinical pharmacological refinement and testing. Preclinical studies include assessing drug bioavailability, metabolism, and toxicity; effects on known biological targets (e.g., receptor binding); and performance in various animal models of pathology. After sufficient data are obtained in animal studies, drug testing in humans can begin. In the United States, human clinical trials are divided into four phases. Phase I involves testing multiple doses of a drug for bioavailability, pharmacokinetics, and side effects. Phase II studies are dose-finding studies in patients with a given disorder. They can be open-label or double-blind trials. Phase III generally includes pivotal double-blind trials for demonstrating efficacy and safety/tolerability. Phase IV trials, which take place after a drug has received U.S. Food and Drug Administration (FDA) approval and is on the market, are conducted to help clarify potential uses of the drug.