The Nevada State Athletic Commission randomly tested Sonnen last month while he was in Las Vegas to attend a UFC news conference. Results confirmed the presence of two illegal substances, anastrozole and clomiphene.
NSAC chairman Francisco Aguilar confirmed the test results to ESPN.com on Tuesday. UFC officials also acknowledged the situation.
"UFC officials acknowledge irregularities in a recent random test," the promotion stated. "Chael is planning on addressing the matter this afternoon."
Sonnen (28-14-1) confirmed the positive test on Fox, which he serves as a UFC analyst.
The substances Sonnen tested positive for are classified as antiestrogenic. Anastrozole is used to treat early stages of breast cancer. Clomiphene is used in cases of female infertility.
Sonnen likely will face a suspension in the wake of the test, which he plans to appeal.
Sonnen, 37, has stated he was diagnosed with hypogonadism in 2008, resulting in a low level of testosterone. He had been approved the use of testosterone-replacement therapy previously in Nevada.
The NSAC banned TRT in combat sports in February, however, which forced Sonnen to abandon treatment. Shortly after the ban, Sonnen stated he would have to find alternative methods to raise his testosterone or retire.
On Tuesday, however, Sonnen made no mention of retirement and defended his use of the two banned substances.
"These are not performance-enhancing drugs," Sonnen said. "They are not steroids of any kind. It's very tough to be caught in this. They changed the rules. I have to comply with rules. However, there is a transition period. I couldn't have been more open or transparent. Anybody I could talk to about this, I did."
According to sources with the NSAC, Sonnen never informed the commission he was using the two substances. Sonnen admitted that later in the interview.
"I wanted to deal with UFC officials. I had no opportunity to go before the commission. I had not spoken to them," Sonnen said.
Sonnen will appeal the results of the test, apparently based on his belief that anastrozole and clomiphene are prohibited only during competition. According to the World Anti-Doping Agency banned substance list, however, both are prohibited "at all times."
"If this was game day, I'm not making any statements that I'm making today," Sonnen said. "On game day, you have to come in right.
"Yes, (the commission can test randomly), but there is a distinction drawn. It doesn't matter if its the NCAA, the IOC or this commission, there has always been a distinction between game day and out of competition. And in out of competition, if you're not on a steroid, that is acceptable."
UFC president Dana White said he was not surprised by the positive test.
"I'm not shocked. This has been lingering," White said. "This stuff has been legal. TRT was legal. Now the (NSAC) said it's illegal. Now it's got to go away, but there's going to be side effects of stopping this thing cold turkey. It just doesn't work that way.
"When it went away, they didn't do a very good job of figuring out how to get these guys off it. When you get off it, you don't just go cold turkey. If they don't take estrogen blockers, you can have side effects. Chael Sonnen is at this point in his life, in his career, where he got married and is trying to have a children. He's on medicine to get fertile. His wife is pregnant, so it worked. This is between Chael and the commission, but the rules should have been laid out better when they said it's over."
Sonnen was originally scheduled to fight Wanderlei Silva at UFC 175, which will take place at the Mandalay Bay Events Center in Las Vegas, but Silva was pulled from the card last month when he refused to submit to a random NSAC test.
Belfort (24-10) is facing a licensing issue in Nevada as well. The Brazilian had also been approved the use of TRT in his last three fights but has since discontinued use. He is scheduled to appear at an NSAC licensing hearing on June 17.
This is the second time Sonnen has technically failed a drug test. In August 2010, he tested positive for an elevated testosterone-to-epitestosterone ratio prior to a UFC middleweight title fight against Anderson Silva in Oakland, California. The California State Athletic Commission suspended Sonnen for one year.
Sonnen appealed the suspension in December 2010, during which he revealed his medical condition and admitted use of TRT. The sentence was reduced to six months.
In May 2011, the CSAC suspended Sonnen indefinitely on suspicion of perjury during that December appeal. That suspension officially ended June 29, 2011.