用人體做實驗，弄清新藥或治療方法的安全性和功效。一般而言，臨床實驗分為3個（有時4個）階段：第一階段評估藥物的安全性；第二階段搞清藥物的最佳藥效和劑量；第三階段（病人數量最多的階段）搞清藥物的臨床藥效。有時還有第四階段，這期間藥物的長期效果在接受藥物的病人中被監測。臨床實驗是新藥開發過程中耗時最長，費用最高的部分，一般要花費5千萬到2億美元和3到5年時間。

 
Studies performed on humans to determine the safety and efficacy of new drugs or therapies. Typically, clinical trials are divided into three (and sometimes four) phases: phase I trials evaluate the safety of the drug; phase II trials determine the optimum efficacy and dosage of the drug; and phase III trials (the largest phase in terms of number of patients) determine the clinical efficacy of the drug. Sometimes a phase IV (post-approval) trial is also pursued, where the long-term effects of the drug are monitored on those receiving prescriptions. Clinical trials are the longest and most expensive part of the process of creating a new drug, typically costing $50-200 million and taking 3-5 years.