Clinical trials that enroll millions of patients in tests of experimental drugs and medical devices get scant government oversight, according to a watchdog report released Friday.

Over a six-year period, the Food and Drug Administration inspected just one of every 100 trial sites, raising questions about the agency's ability to ensure the safety of study participants, according to the report by Health and Human Services Department inspector general Daniel Levinson.

Hampered by the lack of a comprehensive catalog of clinical trials, the FDA is unable to even identify all trials, the estimated 350,000 study sites and the institutional review boards that oversee each study to ensure they meet scientific, ethical and legal obligations, the report says.

The FDA relies on just 200 inspectors to police human studies of drugs and devices. The inspections they perform focus more on ensuring the accuracy of trial data than on verifying the measures put in place to protect the study participants, the investigation found.

Even when inspectors do turn up serious problems, their findings are frequently downgraded by senior officials and almost never followed by inspections to see whether the issues have been resolved, according to the report. In the case of the FDA's drug office, 68 percent of inspector recommendations that the agency take regulatory action, typically in the form of a warning letter, were downgraded.

The FDA found serious problems at test sites 348 times between 2000 and 2005. But only 26 investigators were disqualified from conducting further clinical trials, and data were disqualified just twice, according to the report.

The FDA told the inspector general that inspections make up only one part of its efforts to ensure human subject are protected. The agency sees its review of study protocols before they get under way as the most important step in protecting participants.

The FDA oversees the safety of companies' drug or medical device trials, while the Office for Human Research Protections does the same for federally financed tests. There is no federal monitoring of privately financed, noncommercial trials.