Johannesburg - Ludwick Mamabolo could have taken the banned methylhexamine (MHA) stimulant only the night before or the day of the Comrades Marathon, sports scientist Ross Tucker said on Wednesday.
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"MHA is a stimulant that clears quickly in the system," he said.
"He (Mamabolo) could only have taken it the day before... or even just after reaching the finishing line."
The SA Institute of Drug-Free Sport (SAIDS) announced on Tuesday that Mamabolo, 35, who won the 89km race in Durban on June 3, had tested positive for the substance, which carries up to a two-year suspension from competition.
Mamabolo has reportedly proclaimed his innocence, insisting the substance was found in an energy supplement he had been taking for years.
In Tucker's book, the Science of Sport released two years ago, he called MHA the 2010 drug of the year, with 30 athletes in the world testing positive for it in that period.
The World Anti-Doping Agency officially banned MHA - which can reduce fatigue levels in a runner - in 2009.
Included among the sportsmen who tested positive were Springbok rugby players Chilliboy Ralepelle and Bjorn Basson, who were later exonerated.
Tucker felt Ralepelle and Basson were fortunate to have been protected by the SA Rugby Union "system".
"They were lucky because they were protected by the system, while an athlete is strictly liable," he said.
Another South African rugby player, Cheetahs flyhalf Johan Goosen, received a three-month ban in 2010 after testing positive for the same substance.
Goosen pleaded guilty and stated that he had unknowingly taken the stimulant.
While it is rare for a B-sample to produce different results to an A-sample, Mamabolo is still within his rights to ask for a second test.
Should his B-sample test positive, he could be asked to give reasoning that would help reduce the length of the likely ban.