On Saturday morning, images surfaced of Smith holding a bag of white powder, while a video showing the 21-year-old at a party was also circulating on social media.
Smith and Bulldogs chief executive Ameet Bains confirmed the midfielder had been shown with an illicit substance, in the months after the Bulldogs' 2021 grand final loss, but did not say whether he had taken it.
The Bulldogs and the AFL's integrity unit are investigating the content and how it emerged on social media.
"I have recently been made aware of the presence of social media content that shows me with an illicit substance," Smith said in a statement on Saturday.
"In confronting this, the temptation is to speak in vague generalities and avoid the issue. However, I am determined to learn the lessons of my past and be a better person in the future.
"After much reflection, I am now in a position to confess to indulging in behaviours in late 2021 of which I am deeply ashamed.
"I do not have an excuse for those behaviours, however the state of my mental health over that period post-grand final dramatically deteriorated, and I spiralled out of control leading to poor decision-making and actions at the time.
"All of this resulting in the leave I took from football at the commencement of pre-season training late last year."
Smith "unreservedly apologised" to his family, teammates, the Bulldogs, the AFL and both the club's and his own sponsors and said he fully owned the mistakes he'd made.
The AFL said the Bulldogs and Smith's management had alerted them to the social media content on Saturday.
"The AFL will interview Smith in the coming days in relation to the emergence of the content on social media and the context surrounding it," the league said in a statement.
"An outcome of that investigation will be communicated in due course."
Smith, who is out of contract at season's end, shot to cult-hero status last year, with his hot form and trademark mullet making him one of the most recognisable players in the league.
But he has struggled with the attention at times and took a leave of absence last December to deal with personal issues.
Bulldogs chief Bains said Smith had recently dealt with "many challenges and pressures off the field" and stressed while that didn't excuse his behaviour, the club would continue to support him.
"We are extremely disappointed to have observed social media content showing Bailey Smith with an illicit substance," Bains said.
"This content was captured after the 2021 AFL season, while the players were on personal leave.
"We absolutely do not condone our players being involved with illicit substances and firmly believe players are well-educated on what constitutes appropriate behaviour."
AFL Players' Association chief executive Paul Marsh said the players' union would support Smith and was confident the Bulldogs would do the same.
Smith is currently serving a two-match ban for headbutting Geelong's Zach Tuohy, while the Bulldogs have the bye this week.
