Hamburg  - Olympic cycling champion Tyler Hamilton of the United States retired on Friday after admitting to have failed another doping test.

Hamilton, 38, told the cyclingnews website that he tested positive for the steroid dehydroepiandrosterone (DHEA) at an out-of-competition test in February.

Hamilton said he was aware that an over-the-counter medication he used to treat depression contained the banned substance. He said he used the medication after his mother was diagnosed with cancer.