Essendon have asked the AFL and ASADA to launch an investigation after the club discovered "concerning" information about supplements taken by their players last year.
The club revealed on Tuesday that they had received information in the past 48 hours about supplements given to their players during the 2012 season.
Bombers president David Evans says the information they uncovered was "slightly concerning".
"We have moved quickly to call the AFL and ASADA to ensure a clean bill of health," Evans said.
"We believe we have acted in a sensible and proper way.
"This is a minefield ... we've received information that's concerned us."
Fairfax and News Ltd are reporting that players were asked to sign waiver forms pertaining to treatment they received.
The club did not reveal specifics about how many players took substances or what the substances were.
Essendon's sports science boss Stephen Dank was sacked by the club late last year.
Coach James Hird said he is "very disappointed and shocked" by the news, adding he's confident the club had done nothing wrong.
Channel 9 is quoting a former Essendon player, who says the club knew it was pushing the boundaries with its fitness program.
"From what they were saying, it was right on the borderline of what they were going to give us," he said.
" ... it does seem very odd the type of stuff we were taking.
"They admitted to us it was right on the edge of the levels you could be taking."