UFC Light Heavyweight James Irvin tested positive for methadone and oxymorphone according to the Nevada State Athletic Commission. Both are banned abuse substances and unless Irvin wins an appeal, he will be suspended for 6-12 months and could be forced to surrender all or part of his $20,000 purse.
Irvin was coming off of a broken foot injury in May that caused him to miss a scheduled bout with Rashad Evans (who later got injured himself). If I had to guess, I'd say Irvin was taking the oxymorphone following the foot injury, got slightly addicted, took the Silva bout on short notice (never mentioning he was still taking pain medication to the UFC), then tried to use the methadone to wean himself off before the urine test. Methadone is not prescribed all that often anymore for pain, but it remains in use for opioid withdrawal. Methadone, which itself is an opioid, has a longer shelf life within the body. This results in milder withdrawal symptoms than opioids like heroin, which leaves the body quickly. When an opioid addict wants to get better, the treatment specialists basically switch them over to methadone then eventually taper it off. Irvin probably never got to the point, though, because the oxymorphone was still in his system.
In a sport where fighters put their bodies through hell, I find it hard to criticize someone taking pain medication. He had a legitiment injury and then took a bout on short notice. If anyone is at fault here it is the UFC for not properly checking him out before booking the fight.