Olympic gold medalist Tyler Hamilton, who was banned for two years for doping in 2005, is to retire after testing positive for a banned antidepressant.
The positive test could bring a ban of eight years for the 38-year-old rider, who is suffering from depression.
"I took a banned substance," said Hamilton. "I accept the consequences. You make mistakes in your life and I accept the penalty like a man."
He won gold at Athens in 2004 but the win was clouded by the doping scandal.
Cycling is just a sport, racing your bike from Point A to Point B. What I'm going through is so much bigger
Hamilton was allowed to keep the gold medal because his "B" sample could not be tested, but a month later, he tested positive again.
Hamilton said he has been fighting depression for many years, brought on largely by a divorce and his mother's struggle with breast cancer.
He was clinically diagnosed in September 2003, a year where he won a stage of the Tour de France.
"This isn't about a test," added Hamilton. "It's a bigger issue. It's a disease that I'm going through, that my family has gone through, that I need to take care of.
"Cycling is just a sport, racing your bike from Point A to Point B. What I'm going through is so much bigger."