Arsenal are taking a report striker Alexandre Lacazette inhaled nitrous oxide from a balloon "seriously".
An article in the Daily Star showed the France striker pictured with a balloon in his mouth and he is said to have sent a video to friends.
An Arsenal spokesperson added that it was "a private matter" that would be "handled internally".
Lacazette has previously been warned about a similar offence of inhaling the substance, also known as laughing gas.
Arsenal recently spoke to all their players after Lacazette was one of four players to break social distancing guidelines during the coronavirus lockdown.
On that occasion, the 28-year-old was too close to someone valeting his car.