Collins, in his third season with the team, tested positive for Growth Hormone Releasing Peptide-2 (GHRP-2), the league said.
Collins told ESPN’s Adrian Wojnarowski that he will appeal the suspension.
“First I want to apologize to my teammates, the Hawks organization, our fans, partners and community as a whole for this situation,” Collins said in a statement to Wojnarowski. “I understand the impact this matter has on what we are trying to achieve together this season, and I am incredibly frustrated and disappointed in myself for putting all of us in this position.
“I have always been incredibly careful about what I put in my body, but I took a supplement, which, unbeknownst to me, had been contaminated with an illegal component. I plan to appeal my suspension in arbitration so I can get back on the court as soon as possible and continue to contribute to our 2019-20 campaign.”
Hawks president of basketball operations/general manager Travis Schlenk said in a statement that the organization was “disappointed” to learn of Collins’ violation of the anti-drug policy.
“We hold ourselves and each of our players to a high standard, and we are committed to supporting John as he learns from this setback and continues to grow as both a player and a person. Head Coach Lloyd Pierce and I have both talked to him, and we believe that he is truly remorseful for his actions. We will provide John with support on and off the court while we look forward to his return to the team,” he said.
Collins is averaging 17 points in 32.2 minutes per game this season and leads the team with 8.8 rebounds per game.
His suspension will begin with Tuesday night’s game against the San Antonio Spurs.
Collins is the third player to receive a 25-game suspension for a drug violation this season.
Phoenix Suns center Deandre Ayton was suspended after testing positive for a diuretic, and Brooklyn Nets forward Wilson Chandler was suspended for testing positive for Ipamorelin, a drug that increases the release of growth hormone.