Hunter told Showtime that he witnessed the Argentinian taking the substance, which is said to improve physical performance, as Matthysse gloved up in preparation for the main event at the Hard Rock Hotel in Las Vegas.
At this point (11:22pm PT/4:22am UK), it is not known whether Dallas and Hunter are willing to go ahead with the fight after their protest as officials backstage investigate the claims.
Editors update (11:46pm PT/4:46am UK): The fight between Matthysse and Dallas will go ahead as planned.
