Silva, who on Saturday beat Nick Diaz at UFC 183 after more than a year on the sidelines due to injury, was found to have Drostanolone metabolites in his system after a January 9 test, the results of which the UFC say were only available after the weekend's fight.
A UFC statement on Silva read: "On February 3, 2015, the UFC organization was notified by the Nevada State Athletic Commission that Anderson Silva tested positive for Drostanolone metabolites on his Jan. 9 out of competition drug test. UFC’s understanding is that further testing will be conducted by the Commission to confirm these preliminary results.
"Anderson Silva has been an amazing champion and a true ambassador of the sport of mixed martial arts and the UFC, in Brazil as well as around the world. UFC is disappointed to learn of these initial results.
The UFC has a strict, consistent policy against the use of any illegal and/or performance enhancing drugs, stimulants or masking agents by its athletes."
There are no public records of Silva failing any previous tests, and as such he faces a nine-month suspension if this is found to be his first offence.
In a further blow, the UFC also announced that Diaz had failed a post-fight drug test, with the 31-year-old having tested positive for elevated levels of marijuana metabolites. Diaz has twice previously tested positive for marijuana, in 2007 and 2012.
The UFC said of Diaz: "The UFC has been notified by the Nevada State Athletic Commission that Nick Diaz has tested positive for marijuana metabolites following his fight with Anderson Silva at UFC 183 on Jan. 31 in Las Vegas, Nev. The UFC has a strict, consistent policy against the use of any illegal and/or performance enhancing drugs, stimulants or masking agents by our athletes.
"As a result of his positive test, Diaz has been informed that he has violated the UFC Fighter Conduct Policy and Promotional Agreement with Zuffa, LLC. The UFC organization will fully respect the Commission’s final decision relating to Diaz at a disciplinary hearing set for February 17."