Jarrod Mullen will attempt to have his four-year ban downgraded at the anti-doping tribunal.
The 29-year-old was stood down in January after allegedly testing positive to Drostanolone.
His B-Sample also returned positive, but he will fight the suspension.
"He has opted to take the matter to the anti-doping tribunal," a NRL spokesman said.
His case is set to be heard early next month.
He played 211 matches for the Knights and one State of Origin for NSW but has missed many games over the past few seasons with injury.
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