The American cyclist Tyler Hamilton has tested positive for a banned substance and consequently quit the sport.
The 2004 Olympic time trial champion says he bought an over-the-counter homeopathic anti-depressant containing the steroid DHEA, which is on the World Anti-Doping Agency list of banned substances.
Hamilton, the US national road champion, served a two-year ban for blood doping in 2004.
A second doping offence is subject to a ban from eight years to life, according to the WADA code.
