Greenwood, 18, appeared to inhale nitrous oxide from a balloon in a video, reportedly filmed a number of weeks ago, which was shared on social media platform Snapchat.
“I have now been made aware of the health risks associated with this practice and accept that even trying it, as shown in these historical pictures, was poor judgement on my part. I strongly urge others not to follow my example,” he said in a statement as quoted by Sky Sports.
“As an 18-year-old I am learning all the time, however, this week I have also learned that I will be judged to a higher standard because of my career as a footballer and I must respect that in future. I am determined to repay the faith shown in me by my manager and coaches.”
Greenwood and Manchester City midfielder Phil Foden were sent home from the England squad last week after they flouted quarantine rules while on international duty in Iceland.