Newcastle Knights halfback Jarrod Mullen is facing a lengthy ban after a B-sample returned positive for a banned anabolic steroid.
Mullen was provisionally suspended by the NRL in January following a positive A-sample for Drostanolone, which is prohibited by the World Anti-Doping Agency (WADA), for a test undertaken on November 29.
A league statement read: "The NRL today issued Newcastle Knights player Jarrod Mullen with a Notice of Alleged Anti-Doping Rule Violations under the NRL's Anti-Doping Policy.
"This follows advice from the Australian Sports Anti-Doping Authority (ASADA) that Mullen's B-sample has tested positive for an anabolic steroid, Drostanolone.
"The Notice alleges that Mr Mullen breached the Policy by the Presence and Use of Drostanolone, which is prohibited by the World Anti-Doping Agency (WADA) and the NRL's Anti-Doping Policy.
"Mullen was tested by ASADA on 29 November, 2016 and has been provisionally suspended from participating in any WADA compliant sport, including Rugby League, since 17 January 2017 after his A-Sample tested positive for Drostanolone.
"Under the NRL's Anti-Doping Policy, Mullen now has the opportunity to respond to the matters raised in the Notice of Alleged Anti-Doping Rule Violations."
Knights CEO Matt Gidley added via a club statement: "We obviously remain disappointed with what has transpired but we will continue to respect the process and monitor Jarrod's welfare."
Mullen, 29, has played 211 matches since making his Knights debut in 2005 and is an important member of the first team at the club, who finished bottom of the NRL ladder last season.