Jarrell Miller said he “messed up” and has apologised after a failed drugs test saw his June fight against Anthony Joshua called off.
Miller was due to meet British IBF, WBA and WBO heavyweight champion Joshua, 29, at Madison Square Garden on 1 June.
He was denied a licence for the New York bout after an “adverse finding” from a sample taken on 20 March.
Then after a second adverse finding was reported on Friday, the 30-year-old American admitted: “I made a bad call.”
In a video message posted on Instagram, Miller said: “I messed up. There were a lot of ways to handle the situation, I handled it wrongly and I’m paying a price for it.
“I messed up a big opportunity and I’m hurting on the inside. My heart is bleeding right now – I hurt my family, my friends, my team, my supporters.