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.
Pierce insisted to reporters on Tuesday that his first concern is helping Collins "get through this, not as a player but as a person,'' but the coach likewise acknowledged that Collins let the team down.
"Of course it's disturbing, that this is something you hope you can prevent, and we hope we can educate him on that, but he made a decision, or someone made a decision and somehow we got to this point," Pierce said. "It's our job to get to the information, to keep all our guys from making this decision or being in this position again, but it hurts our team."
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 began Tuesday 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.
The Associated Press contributed to this report.