Andy Ruiz Jr has posted a bizarre video which shows him injecting a fellow boxer in the backside with vitamin B12.
The heavyweight world champion is shown performing the procedure in the gym as he prepares for his rematch with Anthony Joshua in December.
As he injections the substance into 19-year-old fighter Anthony Garnica, Ruiz says: "Guys, if you didn't know I'm not just a boxer, I'm a doctor as well, I give shots for free."
Out of shot, someone can be heard saying the injection is vitamin B12.
Such shots are a common way of preventing or treating a deficiency of the vitamin and are normally injected into a muscle.
It is not clear if Garnica has a deficiency but there is no suggestion Ruiz has acted improperly.
The Mexican-American stunned the boxing world in June when he dropped Joshua four times before stopping him in the seventh round.
Ruiz only stepped in at six weeks' notice to replace drug cheat Jarrell Miller who tested positive for three banned substances.
Ruiz will put his three newly-won heavyweight belts on the line in Saudi Arabia later this year in the rematch.
And in a photo posted last week, the champion looks to have slimmed down ahead of his sequel with the Brit.
Ruiz was pictured sitting on the ring apron with fellow heavyweight Michael Hunter who will face Alexander Povetkin on the Saudi Arabia undercard.
The champion also explained what he said to Joshua in a recent face-off during their promotional tour.
He said: "No, I don't plan on killing nobody, God wouldn't want that, but I'm sure he wants me to win and prove everyone wrong.
"Everybody still doubts me, saying it was God's lucky punch but I told Joshua in his face that it was a punch from God because I've been praying and wishing for this opportunity my whole life and I took advantage of it."
