The Olympic and professional sports world was shocked last month when the U.S. Anti-Doping Agency reported that a new undetectable steroid -- THG -- had been created that allowed athletes to pass doping tests.

"Following the recent developments regarding the anabolic steroid tetrahydrogestrinone (THG), the International Olympic Committee and the World Anti-Doping Agency have advised all stakeholders that THG is a steroid contained in the present and next year's List of Prohibited Substances," WADA and the IOC said in a statement on their websites.

"Gestrinone is namely included on the List of Prohibited Substances and THG, which has a similar chemical structure, therefore must be considered

"THG will be analyzed as part of the steroid class of substances both in competition and out of competition analyses."

European sprint champion Dwain Chambers admitted last month testing positive for the banned anabolic steroid but has denied taking the substance "wilfully".

Four U.S. athletes have also tested positive for the drug, USA Track and Field has said.

Tetrahydrogestrinone is a specially designed anabolic steroid which has been tweaked by chemists to make it undetectable under normal testing.

Anabolic steroids stimulate the muscle and bone cells to make new protein. They increase muscle strength and encourage new muscle growth, duplicating the effects of the male sex hormone testosterone, thereby allowing the athlete to train harder for longer periods of time.