The Food and Drug Administration should have the power to require that drug makers conduct studies on the safety of prescription medications already on the market, congressional investigators recommend.

Drug makers often promise timely follow-up studies, but they often delay going ahead, the agency said. As a result, the FDA can lack useful data in determining whether an approved drug really is safe, according to a report being released Monday by the Government Accountability Office.

Investigators also are urging Congress to change how the FDA tracks potential concerns about drug safety, makes major decisions and settles internal disputes, especially between the two offices that now assess drug safety.

“FDA lacks clear and effective processes for making decisions about and providing management oversight of, post-market safety issues,” the report says.

In a response included in the report, the FDA said the conclusions were reasonable and consistent with actions the agency had planned or already begun.

Sen. Charles Grassley, R-Iowa, and Rep. Joe Barton, R-Texas, requested the investigation after several recent drug safety cases, including the withdrawal from the market of the pain killer Vioxx in 2004.

“This report provides solid evidence that everything is not all right at the FDA and calls for long-overdue reform. The FDA’s problems are systemic and cultural, not isolated or easily fixed,” Grassley said.