Jeffress' agent, Joshua Kusnick, cound not name the substance for which Jeffress tested positive, but said it was not a performance-enhancing drug. Jeffress was suspended 50 games in 2007 for "a drug of abuse" and later admitted to marajuana use.
Major League Baseball could announce Jeffress' latest suspension as early as Monday.
"Jeremy is extremely remorseful for what happened and will be voluntarily checking himself into a rehab facility to deal with his problem," Kusnick said. "Jeremy is sick and needs help. We are here to support him in getting his life back. Right now, the most important thing is to help him get his life back, and baseball will take care of itself.
"He is sorry for letting everyone down who believes in him."
Under the Minor League Drug Treatment and Protection program, players who test positive a third time face a lifetime ban.
Jeffress was the Brewers' 18th overall pick in the 2006 First-Year Player Draft and is ranked as the organization's fourth-best prospect, according to Baseball America. He began the season with Double-A Huntsville, struggling with a record of 1-3 and 7.57 ERA, before being demoted to high Class A Brevard County last month.
In five starts in Brevard County, he went 2-1 with a 2.18 ERA.
Adam McCalvy is a reporter for MLB.com. Cash Kruth is an associate reporter for MLB.com This story was not subject to the approval of Major League Baseball or its clubs.