A report by The Athletic has revealed that the 36-year-old served a six-week FA suspension for failing a random drugs test in 2018.
Murphy has apologised for an “isolated incident” which occurred in the off-season, claiming he had made a “bad decision while on a night out.”
The former Republic of Ireland international did not play for Forest between October 6 and December 8 last season – and having scored four goals in his first eight outings under Aitor Karanka, added only two more in 24 appearances before the end of the season.
Wanderers have confirmed the story – underlining that it had no effect on his time in the North West, which began with a debut at Portsmouth on September 28, 2019, after a short spell out injured.
“We are aware of the events which took place prior to Daryl’s arrival at the club,” a statement read.
“Daryl served a suspension and has expressed regret over the incident, which occurred out of competition.
“Daryl is a valued member of the first team squad and the club will be making no further comment on this matter.”