On July 24, 2014, it was announced that Diaz had signed a three-fight contract to return to the UFC. Diaz faced Anderson Silva on January 31, 2015 at UFC 183. He lost the fight by unanimous decision. A few days after the fight, the UFC revealed that Diaz had once again failed his post-fight drug test for marijuana metabolites, and Silva tested positive on January 9 for drostanolone, androstane, oxazepem, and temazepam in pre-fight drug screening. On August 13, after several reschedules, Silva's disciplinary hearing was held to decide on the subject. Silva's defense argued that a tainted sexual enhancement supplement was the root of the two failed tests for drostanolone and also appealed to mistakes in the NSAC testing procedures, pointing to a pair of drug tests, one on Jan 19 and one after the fight, which Silva passed. He admitted to using both temazepem and oxazepem, benzodiazepines, the night prior to the fight as therapy to control stress and help him sleep. Silva's team was unable to explain the presence of androsterone in the Jan 9 test. The commission rejected the defense and suspended him for one year retroactive to the date of the fight, as the current guidelines were not in effect at the time of the failed tests. Silva was also fined his full win bonus, as well as 30% of his show money, totaling $380,000. The result of the bout was changed to a no contest. At that point, Diaz had accumulated a UFC record of 7 wins, 6 losses and 1 no contest, with no successful challenges for a title.