Mamabolo shocked the nation when he tested positive for the banned drug methylhexaneamine, which is commonly found in performance-enhancement supplements and energy drinks.

Ross Tucker, of the Sports Science Institute, said it was extremely rare that an athlete's B sample could test negative after the A sample was positive.

"It is highly unlikely that he would fail one test and pass the next one," Tucker said.

SA Institute for Drugfree Sport CEO Khalid Galant agreed, saying "something like that is virtually unheard of, but not impossible".

Strict World Anti-Doping Agency laws dictate that the athlete is 100% liable for what goes into his or her system. Therefore, Mamabolo could be found guilty even if he was able to prove that the substance got into his system by "accident", Tucker said.

"His first option is to have his B sample tested and hope that it comes back negative," said Tucker. "The second option is that he has to prove that the stimulant got into his system via a contaminant that he was taking innocently and that is very difficult to do.

"Even if it gets into his system by accident, he will still end up being liable for it ... Ignorance is not a defence."

He said the ball was now in Mamabolo's court: "The burden of proof now lies with the athlete, who has to show where the substance came from and that it wasn't his fault. It is not the same as in criminal cases, where the burden of proof is on the prosecutor.

"Mamabolo has said he is innocent, and I'm pretty sure that he didn't take [the drug] on purpose, but that doesn't matter. If found guilty, he is still going to get banned."

In 2010, Springbok rugby stars Chiliboy Ralepelle and Bjorn Basson received three-month bans after testing positive for the same substance despite proving that they had not taken it intentionally.

Wada tested the top 10 finishers at this year's Comrades.

Mamabolo stands to lose his Comrades title and R300000 in prizemoney.

He also faces a ban from the sport of up to two years.

His supplement sponsors 32GI, have, however, denied that their products were responsible for the positive test.

Mark Wolff, a director at 32GI, claimed that it was impossible Mamabolo could have ingested the stimulant from one of their endurance products because eight of the top 10 men finishers ran on the product.

"We do 17 of the Mr Price running team athletes. In fact, at the Comrades, 11 of the 20 gold medallists [men and women] used our product.

"Our products are clean, natural and child-friendly. If the stimulant was in our product you could have had 11 positive drug tests in the Comrades.

"Mamabolo is an intelligent guy and something like this wouldn't have given him a benefit in the ultra-marathon. It was most likely taken accidentally, but you never know with athletes."