The Food and Drug Administration said yesterday it is setting up a new system for identifying potentially dangerous and mislabeled dietary supplements, and it promised to aggressively go after companies that sell misleadingly labeled products.

Responding to criticism that the government has reacted too slowly to the dangers posed by popular supplements such as ephedra and androstenedione -- both taken off the market in the past year because of health concerns -- the FDA outlined a new regulatory scheme for overseeing the fast-growing supplement industry.

In addition to upgrading its process for collecting "signals" of possible harmful effects in people using supplements, the FDA also set its first standard for determining whether the claims on labels are properly substantiated. From now on, supplement makers will have to show "competent and reliable scientific evidence" to back up their claims, it said.

An estimated 1,000 dietary supplement companies sell 29,000 products, but since passage of the 1994 Dietary Supplement Health and Education Act, the industry has been largely free of government oversight.

High-profile deaths associated with the diet and bodybuilding supplements ephedra and "andro" put pressure on the FDA to use its limited regulatory authority, and yesterday's announcement was an effort to expand its mandate.

"These initiatives refine the direction the agency is taking to regulate dietary supplements," acting Commissioner Lester M. Crawford said.

"We now have a clear road map to share with the dietary supplement industry, while at the same time giving consumers a higher level of assurance about the safety of dietary supplement products and the reliability of their labeling," he said in a statement.

The FDA said it sent letters to major dietary supplement retailers warning them against selling mislabeled products and threatened enforcement action if they do. Two weeks ago, the agency sent out eight letters to distributors making what it considers to be unsubstantiated weight-loss claims for supplements sold over the Internet, officials said.

Under the 1994 act, most dietary supplements do not need FDA review or approval before going on the market, but some new ingredients require a pre-market report on potential safety problems. As another part of its new initiative, the FDA will hold a public meeting on Nov. 15 to discuss what evidence should be required for these ingredient notifications.

The proposals were criticized by Janell Mayo Duncan, legislative counsel for Consumers Union, which has followed the issue since the mid-1990s. This summer, she said, her group identified 12 dangerous supplements that should be taken off the market immediately, but the FDA proposals do little to make that happen.

"What we need now is action, not more discussion of methods and procedures," she said. "The problems have been known for years, and the rulemaking process has been going on for years."

Duncan said the FDA effort to increase surveillance for harmful side effects is worthwhile, but the proposal would still leave adverse event reporting up to the companies rather than making it mandatory, as it is with prescription drugs.

The FDA announcement was generally welcomed by Annette Dickinson, president of the Council for Responsible Nutrition, a Washington group that represents many supplement makers.

"We think it's time for the FDA to systematically implement" the 1994 law, she said. "We think this will demonstrate that [the law] works when implemented properly."

She said companies that her group represents have experienced greater oversight from the FDA in the past year, and the agency has rejected new-ingredient notifications filed by many of them. She said she hoped the new FDA advisory would make the process of new-ingredient notification more predictable for companies.

The FDA's Lester M. Crawford said the guidelines will reassure consumers.