France 400m hurdler Naman Keita has reportedly failed a drugs test from a pre-World Championships sample.
Keita won a bronze medal at the Olympics
Keita, 29, has cited a food supplement he ordered online to help a sore stomach muscle as the possible cause.
"I found out about it from the FFA president (Bernard Amsalem) on Friday night," said Keita.
"It was an out of competition control made on August 20 in Wakayama (the training camp used by France to prepare for Osaka)."
The IAAF has yet to confirm the positive test, but if it emerges that Keita did fail and the analysis of the B-sample confirms the result, then he risks a two-year ban.
"I made a huge error," added the athlete. "July was a catastrophic month for me in terms of performances and after the French championships I was suffering a little in my stomach.
No product or food supplement should be taken if it has not been approved by the FFA's medical staff
"I was advised a dietary supplement to help the muscle regenerate and I ordered it off the internet.
"It wasn't my coach, nor my agent, nor anyone from the Federation who suggested doing it. The mistake I made myself.
"After the French Championships I went a little into myself. I didn't want to speak to anyone. I was full of doubt."
The French Athletics Federation (FFA) has emphasised the need for athletes to run any supplements past its doctors.
"The FFA president is very disappointed and would like to state how important it is for athletes not to let just anybody give them advice," said a statement from the FFA.
"No product or food supplement should be taken if it has not been approved by the FFA's medical staff."
Keita went out at the semi-final stage of 400 hurdles at the World Championships.
He won gold with the French 4x400 relay team at the 2003 worlds in Paris.
France left Osaka with just two silver medals and IAAF president Lamine Diack said on the final day of the tournament that the anti-doping programme at the event had unearthed one "adverse result".
A total of 1,060 doping tests - 510 blood tests and 550 urine - were conducted on 926 athletes at Osaka.