Americans should immediately stop using a chemical mix masquerading as a dietary supplement that was blamed for poisoning the livers of at least six people, the government warned Tuesday.

The victims, ages 20 to 32, suffered liver failure or acute hepatitis after taking Lipokinetix for two weeks to three months, the Food and Drug Administration said.

Lipokinetix, widely sold on Internet bodybuilding and weight loss sites, is believed to contain a drug banned a year ago, a potent thyroid hormone, and a chemical once tested as a topical therapy for genital warts that FDA has not approved for internal use.

The FDA urged that anyone who has used Lipokinetix call a doctor if they have any of the following symptoms: nausea, weakness or fatigue, fever, abdominal pain, or any change in skin color.

The stern warning marks the second time in two years that FDA has clashed with the product's manufacturer, Missouri-based Syntrax Innovations Inc., which sells remedies claiming to fight fat, build muscle and boost energy.

In 1999, the FDA ordered Syntrax to stop illegal sales of a thyroid hormone posing as a fat burner. Syntrax fought the order in court but eventually backed down.

Underlying the urgency of the Lipokinetix investigation, the FDA asked thousands of health care providers Tuesday to review medical records for cases of unexplained hepatitis - liver injury - that might be linked to dietary supplements.

But in a highly unusual move, Christine Taylor, the FDA's dietary supplements chief, refused to say whether any of the six patients believed to have been injured by Lipokinetix had died or needed liver transplants. She said to do so would invade their privacy.

On Tuesday, Syntrax owner Derek Cornelius denied Lipokinetix was dangerous. But Cornelius said he stopped selling it last year because of difficulty in buying some ingredients.

"We sold over 30,000 bottles," he said. "Why, a year after I stopped selling it, did they issue their warning?"

Cornelius charged the FDA also plans to target a newer, similar product the company sells called Adipokinetix, because government agents recently seized a shipment of a chemical it was importing to brew it.

"Unfortunately (Lipokinetix) has been discontinued," reads one Web site. "Check out Adipokinetix for a similar product with a new name. Amazing fat loss and energy."

Should consumers avoid Adipokinetix, too? FDA's Taylor said she didn't know but that the investigation is concentrating on the original Lipokinetix because that is what the injured patients consumed.

Doctors are not required to report suspicious side effects to the government - and studies show most don't - so it can take the FDA months, even years, to determine if a substance is dangerous. To minimize that problem, drugs are studied before they are allowed to be sold, but a 1994 law exempts dietary supplements from most federal regulation, forcing the FDA to prove a supplement is dangerous before taking any action against it.

The FDA said Lipokinetix contains the drug PPA, yanked off the market a year ago because it increases the risk of hemorrhagic strokes; a potent thyroid hormone; yohimbine, a tree bark substance that can raise blood pressure; and sodium usniate, a chemical derived from fungus that has been studied as a skin cream for genital warts.