Double world sprint champion Kelli White has tested positive for a stimulant, Modafinil, which is not currently on the banned list, International Association of Athletics Federations spokesman Nick Davies said on Saturday.
Davies said the 26-year-old American had tested positive after the 100 metres final on Sunday. But the substance is not on the IAAF's list of prohibited drugs.
The spokesman said the IAAF is examining the substance and will make an announcement later on Saturday.
White, the first American woman to win the 100-200 double at the world championships, is scheduled to run in the 4x100 metres relay final on Saturday.
The IAAF said in a statement, "A sample provided by Kelli White, USA, after the 100m final in Paris has tested positive for Modafinil. The IAAF has not yet received the results of the analysis on her sample provided after the 200m final.
"Today, we have received an explanation from the athlete to the following effect: Ms White has been taking the substance on prescription to treat a medical condition that runs in her family.
"As both the nature of the substance and the content of her explanation require further inquiry and investigation, the IAAF has taken the view that bearing in mind the very serious consequences for the athlete of being suspended and being removed from competition at this late stage, the most appropriate course of action is to allow her to compete pending the completion of the necessary scientific and factual investigations."