UFC's Chael Sonnen retires after failed drug test

UFC veteran Chael Sonnen retired from mixed martial arts a day after confirming he failed a recent drug test.

Sonnen's bout with Vitor Belfort at UFC 175 in Las Vegas next month was scrapped after Sonnen tested positive for two banned substances.

"When I woke up yesterday morning, I never would have had the foggiest idea that I was going to get notified that I had a banned substance in my system," Sonnen said on Wednesday night's edition of "UFC Tonight," which he co-hosts. "These are not performance-enhancing drugs, these are not illegal drugs and these are not steroids. They banned testosterone use in Nevada. Now I had to comply with that because I was a user. When you come off this you have to go through a transition phase. We were very transparent about that."

The 37-year-old Sonnen (28-14-1) says he took the substances to ease his transition off testosterone replacement therapy, which was banned by the Nevada Athletic Commission earlier this year.

"My wife and I were having problems with fertility," Sonnen said. "This is a matter of health and being a parent and this is a matter of being able to live in the next chapter of my life. At no point was there an attempt to slip around the system. I want to admit fault and accept full responsibility. Yesterday when this came out, I was mad. I was a fighter and I went in defense mode. However ultimately the rules are the rules and I'm a rules guy, I don't want to break the rules. And ultimately it comes down to the athlete to make sure he's in compliance."

Sonnen (29-14-1) has lost three of his last four fights, including UFC title shots against Anderson Silva and Jon Jones. It was the first fight against Silva that turned Sonnen into a star. He dominated Silva, then the middleweight champion, for more than four rounds only to tap out to a triangle choke in the final two minutes of the last round. Until that fight, Silva had always appeared unbeatable in the cage. Sonnen tested positive for elevated levels of testosterone after that fight. His rematch against Silva in 2012 was among the biggest fights in the UFC's history.

"I want to thank the single most important opponent I've ever had, I couldn't have gotten where I am without the dance partner, Anderson Silva," Sonnen said. "Thank you for the opportunity and the memories. Thank you for the invitation to the barbecue. I want to officially announce my retirement from competitive mixed martial arts."

Sonnen lost all three of his career UFC championship fights. He made his UFC debut in 2005.

He announced his retirement Wednesday on Fox Sports, where he serves as a UFC commentator.