WASHINGTON -- The newly detected steroid that is casting a shadow on Olympic and professional sports is an illegal drug that may pose considerable health risks, the government warned Tuesday.

THG has been sold in the guise of a dietary supplement when it is in fact a drug that lacks federal permission for sale in this country, the Food and Drug Administration said. It is a drug derived from another steroid long banned in athletics, the agency said.

The FDA's official designation of THG as illegal, which had been anticipated since the scandal over the previously undetectable steroid emerged, puts manufacturers on notice that the government will crack down on anyone caught selling it.

It also is the strongest warning yet that using THG is risky. Anabolic steroids can have dangerous side effects, including liver damage, heart disease, anxiety and rage. While little is known about THG's specific effects because it is new, its close chemical similarity to other well-known steroids means it poses the same risks, FDA Associate Commissioner John Taylor said.

"The greatest importance is preventing exposure and trying to nip this in the bud," he said.

U.S. drug authorities first learned about THG, or tetrahydrogestrinone, this summer after an unidentified coach gave them a syringe containing it. THG apparently was designed specifically to be undetectable by the standard test given to athletes.

Now armed with a test, sports organizations are scrambling to re-examine athletes and to decide what penalties to impose for THG use. Four U.S. track and field athletes have tested positive for THG, and Europe's top sprinter has admitted taking it in nutritional supplements that he says he thought were allowed.

Exactly who developed THG is unclear. Dozens of top Olympic and professional athletes have been subpoenaed to testify before a federal grand jury probing a California lab that sells nutritional supplements. Its owner has denied supplying THG, and federal officials, including FDA's Taylor, refuse to comment on the scope of their investigation.

Troubling to lawmakers is that THG apparently was sneaked onto the market in the guise of a dietary supplement. It's not a supplement but an unapproved drug, making any sale or usage illegal, FDA's Taylor said.

Currently, however, "There's nothing to stop another group of folks in another lab from concocting another designer steroid that will circumvent this FDA ruling," said Joe Shoemaker, spokesman for Sen. Dick Durbin, D-Ill.

Durbin is pushing legislation that would give FDA broader oversight over dietary supplements to prevent steroids from being sneaked onto the market. He said he was unimpressed by the agency's move against THG and said it falls short of dealing with similar substances.

"It's sad that it takes a national controversy, a lawsuit and a lot of publicity to bring the FDA around to their core responsibilities," he said in an interview.

Sens. Joseph Biden, D-Del., and Orrin Hatch, R-Utah, introduced similar legislation last week that also would outlaw steroid precursors like androstenedione, popularized by baseball's Mark McGwire.

Anabolic steroids are synthetic versions of the male hormone testosterone. Some are approved by FDA for prescription-only sale to treat certain diseases; athletes use them illegally to bulk up muscle and enhance performance.

FDA's testing of THG shows it was derived by simple chemical modification of gestrinone, a drug used in Europe to treat a gynecologic condition. It is explicitly banned by the U.S. Anti-Doping Agency, which monitors drug use by athletes in Olympic sports. THG also is very closely related to trenbolone, a controlled substance used to bulk up cattle.

THG "is a designer steroid in the truest sense," Taylor told a Senate committee.