The Food and Drug Administration said in an annual report that, as of Sept. 30, 65 percent of the 1,231 "post-marketing" studies that companies had pledged to carry out were still pending.

"That doesn't mean they will never be started," said John Jenkins, director of the FDA's Office of New Drugs, noting that 116 of the 797 studies were committed to during the 12 months ending in September. The clinical trials can take six months to a year to design and launch, he said.

Some studies had been committed to years earlier, but the FDA did not provide a breakdown.

The 797 pending studies represent a slight dip from 812 a year earlier. FDA official Kathleen Quinn said the agency feels that "these numbers show drug companies are taking this thing seriously."

Jerry Avorn, a Harvard Medical School professor and author of "Powerful Medicines," in which he criticizes the FDA's post-marketing system, said the numbers show the system is broken.

"This new information is an embarrassing continuation of similar reports issued by FDA each year on the appalling state of the medication safety studies it has 'mandated' drug manufacturers to perform. It is scandalous that of the supposedly active studies, about two-thirds haven't even been started yet," Avorn said.

But Alan Goldhammer, of the Pharmaceutical Research and Manufacturers of America, said the figures should not be "distorted."

"To be clear, pending does not mean delayed. It does mean, however, that the immense and vitally important tasks of developing research protocols, finding investigators and researchers and even recruiting patients to participate in the study is in process," Goldhammer said.

The FDA said it relies on the so-called Phase 4 studies to gather additional information about a drug's safety, efficacy or use. The outcome of those studies can lead to changes in how a drug is made, prescribed and used. The FDA also can require the studies after it has approved a drug.

The report, posted to the FDA Web site, lists 231 studies as ongoing, 28 as delayed and three as terminated as of Sept. 30. Another 172 studies are listed as completed or terminated, with a final report submitted to the agency.

The report also tallies studies required of biological products, which include vaccines, blood components and transplant tissues. There, of 321 study commitments, 118 -- or 37 percent -- remained pending as of Sept. 30. Another 56 were completed by that date.