Former Blues striker Daryl Murphy served a secret six-week ban early last season after testing positive for cocaine while with Nottingham Forest.
Murphy, 36, now with Bolton, is understood to have served his ban between 6th October and 8th December last year but with the sanction not made public under the FA’s policy of not naming players who have failed drugs tests.
In a statement given to The Athletic, Murphy said: "Firstly, I would like to stress that the taking of recreational drugs is something that I don’t condone whatsoever.
"I served a suspension at the beginning of last season for making a bad decision while on a night out.
"This was an isolated incident which happened out of competition when we had no game. I immediately regretted it.
"I am not proud of what I did but it’s something I have put behind me as I want to concentrate on my football career.”
Murphy had three loan spells with the Blues before signing permanently in June 2013.
The Irish international scored 27 times in the 2014/15 play-off season, in which he was the club's Player of the Year, before departing for Newcastle for £3 million in the summer of 2017.
Elsewhere, ex-Blues defender Tommy Smith has been linked with Championship Huddersfield Town.
The Terriers are said to be leading the chase for Smith, 29, after his the Colorado Rapids opted not to take up the option on his current contract.
However, it’s understood talks regarding revised terms are continuing between the New Zealand international, who has been training with Town’s U23s, and the MLS club.