Clinical trials are research studies that assess a candidate drug's pharmacological properties in humans. These trials are conducted in multiple phases. In phase I, five to ten candidate drugs from successful preclinical studies are tested on a small group of either healthy individuals or patients to evaluate various effects, such as potential toxicity, side effects, pharmacokinetics, and pharmacodynamics. Successful candidates from phase I are tested in phase II on a hundred to three hundred patients to monitor drug efficacy and dosage regulation. Candidates that pass phase II screening are tested in phase III for their safety and efficacy. Phase III trials are double-blind studies conducted on thousands of patients across many centers. After a successful phase III trial, a New Drug Application is submitted to the FDA for marketing approval. Once the drug is FDA-approved, the manufacturer produces and places the drug on the market. Postmarketing surveillance, or phase IV, is done to study the drug's long-term or rare adverse effects.