The Frenchman was pictured with a balloon in his mouth and is said to have sent a video to friends saying he was “at home, chilling and doing balloons”, reported Daily Star.
Lacazette was warned by the club in 2018 when video footage emerged of him and teammates Mesut Özil, Matteo Guendouzi and Pierre-Emerick Aubameyang inhaling the gas, also known as hippy crack, in a nightclub.
“This is a private matter which we are taking seriously. It will be handled internally,” said an Arsenal spokesman.
It is the second misdemeanour in recent weeks for Lacazette, having had to be reminded of his responsibilities last month when he was pictured flouting lockdown guidelines by not social distancing while having his car valeted on his driveway.