Major League Baseball (MLB) and Major League Soccer (MLS) have added THG -- tetrahydrogestrinone -- to their list of banned substances, while the National Football League (NFL) insist they will come down hard on players testing positive for the newly-discovered steroid.

"My position is that it is a steroid and we treat it like we treat steroids," Harold Henderson, executive vice-president and chairman of the NFL management council, was quoted as saying on Thursday.

"They're banned... we test for them and whenever we have a test for them, we take action when we find positives."

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Tetrahydrogestrinone is a banned steroid which had been tweaked by chemists to make it previously undetectable under normal test conditions.

With officials having announced recently that four U.S. athletes have tested positive for the drug, USA Track and Field, the governing body of American athletics, have since adopted a "zero tolerance" policy towards those found guilty.

Professional athletes from a cross-section of sports in north America have been subpoenaed to appear before a grand jury probing the finances of the Bay Area Laboratory Cooperative (BALCO).

ILLEGAL SUBSTANCE

BALCO, a Californian company which makes nutritional supplements, has been identified by the United States Anti-Doping Agency (USADA) as what they believe was the source of THG.

The NFL said last week that THG would now be included in current testing as a banned substance.

Commissioner Paul Tagliabue confirmed on Wednesday, after a meeting of league owners in Chicago, that they are also discussing the possibility of testing previous samples for THG.

The NFL conducts random testing, and issued a stern message to its players after the meeting.

"It's a strict liability policy," Henderson told Thursday's edition of the New York Times.

"You're responsible for what's in your body -- the players are taught that from the minute they come into the league."

Earlier this week, the Food and Drug Administration announced that THG is an illegal substance -- a move which led MLB and MLS to banning the steroid on Wednesday.

"There is simply no room for cheaters in sports," said MLS commissioner Don Garber in a statement.

"Our League refuses to face a future where the performance of our players or the integrity of the world's most popular sport is tarnished in any way."

A spokesman for the National Basketball Association (NBA) confirmed to Reuters that THG was not on its list of banned substances.

The National Hockey League (NHL), which does not conduct random testing, was not immediately available for comment on Thursday.