West Brom have taken action against Saido Berahino after the striker was videoed inhaling nitrous oxide in his car after the Baggies' 3-0 defeat to Manchester United last month.
The video, taken on March 8 and brought to light by the Sunday Mirror, shows the 20-year-old puffing the substance, otherwise known as 'laughing gas', from a balloon before slumping down over his steering wheel.
Inhaling nitrous oxide is not illegal but it can pose serious health risks.
West Brom sporting and technical director Richard Garlick said in a statement released by the club: "I have spoken to Saido about the incident and told him in no uncertain terms that his conduct is not befitting a West Bromwich Albion player.
"Saido knows he has made a mistake. He has apologised for his actions and has assured the club it will not happen again."
Berahino said: "This was very poor judgement on my part and not the right example to be setting.
"At the time, I wasn't fully aware of the serious health risks involved but now I know, I won't be doing it again."
It's not the first time a Premier League footballer has had to apologise for being caught inhaling the substance.
Tottenham defender Kyle Walker was pictured using nitrous oxide days before England's goalless draw with Ukraine in September.
Drug advice website Frank says: "There is a risk of death as a lack of oxygen can occur when using nitrous oxide.
"This risk is likely to be greater if the gas is consumed in an enclosed space or if a substantial amount is rapidly used."