Mullen has returned a positive A-sample for Drostanolone when tested in late November last year.
The 29-year old has been stood down by the NRL under its anti-doping policy after ASADA notified the club and governing body.
Under the NRL’s policy Mullen can either immediately accept the results or have his ‘B-sample’ tested.
“The club is obviously extremely disappointed,” Knights CEO Matt Gidley said.
“The club has a strict governance program, the players are regularly educated and fully aware of the consequences of going outside our governance guidelines.
“From here, there is a formal process we need to follow under the guidelines.
“We need to respect the process and in the interim the club will continue to monitor Jarrod’s welfare.”
Mullen, who was linked to a move to Huddersfield late last year, has spent his whole career at Newcastle.
After debuting in 2005 the playmaker has gone on to make 211 NRL appearances.
Mullen played one Oriigin game for NSW in 2007.