The Newcastle Knights back, who represented New South Wales in the 2007 State of Origin series, had appealed to the NRL's anti-doping body in March after a four-year ban was recommended by the Australia Sports Anti-Doping Authority (ASADA).
The basis of the 30-year-old's appeal was that he had used the drug not for competitive advantage but to help with his recovery from a series of serious hamstring injuries.
The NRL's anti-doping tribunal confirmed the ban on Wednesday, leaving Mullen 21 days to appeal to the Court of Arbitration for Sport.
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"ASADA will continue to work to protect those athletes who make the right decisions when it comes to injury treatment and rehabilitation," ASADA chief executive Ben McDevitt said in a statement acknowledging the ban.
"Ultimately, Mr Mullen has paid a heavy price for his poor decision making."
(Reporting by Nick Mulvenney, editing by Amlan Chakraborty)
