The 20-year-old has tested positive for the steroid stanozolol and has already left the country, Olympic medical commission chief Alexandre de Merode has revealed.
After a meeting of the International Olympic Committee's medical commission, de Merode said the athlete had failed a random test taken a few days after finishing competing.
He said: "The athlete took part in one competition and had a test afterwards, but some days afterwards, and then left because the team estimated that she was not well enough qualified."
Officials
Officials are running out-of-competition tests for the first time at an Olympics.
Before the athletics started, Belarus hammer thrower Vadim Devyatovsky was sent home after testing positive for nandrolone in an out-of-competition test in the Olympic village.
Five competitors, none of them from athletics, have tested positive for banned drugs in competition tests carried out during the Olympics. Four of the five were stripped of medals.
Several athletes were banned from the Games for failing out-of-competition tests before the start.
Legal
The IOC will have to discuss the legal issues involved since the athlete has already left the Games and can therefore no longer be expelled.
"That is the juridical point. We don't know what to do exactly because the person is not here. We have to look to the executive board for what we have to do."
Bulgarian weightlifters Ivan Ivanov, Izabela Dragneva and Sevdalin Minchev, Latvian rower Andris Reinholds and Romanian gymnast Andreea Raducan are the five to have failed competition tests during the Games.
Apart from Reinholds, the others all won medals at the Games.