Recent actions by the FDA that have delayed approval of new indications for already marketed drugs suggest that the agency does not believe that prolonged clinical experience necessarily answers all the concerns surrounding a drug's potential impact on humans.

Although the agency encourages companies to seek approval of marketed products for new indications, it requires such applications to meet the current standards for demonstrating safety and efficacy. Of particular concern are carcinogenicity and reproductive toxicity, which cannot easily be determined with human trials.