Soto, who talked to Chicago beat writers after Thursday's game, a 6-5 loss to the Detroit Tigers, said he found out about the positive test a couple months ago. Soto wanted to be able to address the subject, and decided to do so now rather than wait for the International Baseball Federation to announce the test results.
"This has been bothering him," Cubs assistant general manager Randy Bush said. "We knew he'd handle it directly and head on, and that's what he's doing."
The 2008 National League Rookie of the Year, Soto has been drug tested throughout his career, and this is the only positive test. He continues to be subject to MLB's random testing but nothing additional.
Soto "assured the organization this was an isolated incident and a misstep in judgement that will not be repeated," the Cubs said in a statement.
"I think the world of Geo, we all do in the organization," Bush said. "None of us think any less of him. He's a standup guy, wonderful kid. We love him and support him and he's been anxious to put this behind him."
Soto, who played for Team Puerto Rico in the World Baseball Classic, was batting .227 with six home runs and 21 RBIs this season. He was not in the lineup for Thursday's game, but ended it when he struck out swinging as a pinch-hitter in the ninth.
Keeping this secret must have affected the young catcher, who hit .285 with 23 homers and 86 RBIs last season.
"I think he would never make excuses," Bush said. "That's the kind of guy he is -- but I think it's been bothering him."
"It was a mistake," Soto said, "and I'm sure everybody makes mistakes in their lives. I guess this was my mistake. It's one of those things. I guarantee it will never happen again. I'm not that kind of person. I want to move forward.
"I'm looking forward to this being long gone," he said.