用人体做实验，弄清新药或治疗方法的安全性和功效。一般而言，临床实验分为3个（有时4个）阶段：第一阶段评估药物的安全性；第二阶段搞清药物的最佳药效和剂量；第三阶段（病人数量最多的阶段）搞清药物的临床药效。有时还有第四阶段，这期间药物的长期效果在接受药物的病人中被监测。临床实验是新药开发过程中耗时最长，费用最高的部分，一般要花费5千万到2亿美元和3到5年时间。

 
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.