The U.S. Food and Drug Administration does not know the status of many post-approval studies promised by drug makers and does not make tracking them a top priority, a report released on Friday said.

The inspector general at the Department of Health and Human Services said the FDA should improve its monitoring of the studies by upgrading its tracking systems and asking manufacturers to provide better information.

Most studies are promised voluntarily by drug makers to learn more about benefits or side effects after medicines reach the market. Forty-eight percent of drug applications approved from 1990 to 2004 involved at least one post-approval study commitment, the inspector general’s report said.

Companies are supposed to give FDA annual reports on the progress of their studies. The inspector general said 35 percent of 336 annual reports due in 2004 “were missing entirely or contained no information on open postmarketing study commitments.” The FDA validated only 30 percent of the reports, the report said.

“FDA cannot readily identify whether or how timely postmarketing study commitments are progressing toward completion,” the report said.

FDA reviewers who were interviewed “indicated to us that monitoring postmarketing study commitments is not generally considered a top priority,” the report said.

The FDA cannot force companies to complete the voluntary studies, but the agency said it had taken steps to improve monitoring and make some information public.

FDA critics have complained for years that drug makers often fail to follow through on promised post-approval studies. Some have called on Congress to give the FDA more power to punish drug makers that leave studies unfinished.

In comments submitted to the inspector general, the FDA said it took “very seriously” its responsibility for monitoring post-approval studies.

The agency agreed it could make improvements but disputed the finding that it could not tell the status of many studies. It said it had improved a database of post-approval research and was training staff on confirming information in companies’ annual reports.

The FDA also has hired an outside contractor to make recommendations on ways to make sure companies complete more the studies.