Last week, the musician was arrested in Nashville after allegedly driving under the influence and crashing into a pickup truck, causing the truck's driver to be ejected from the car and killed.
Harrell's lawyer told People, "He was released on bond and immediately went and checked himself into treatment," though declined to say where Harrell was being treated.
Harrell's lawyer did note, though, that the treatment was voluntary rather than court-ordered, with Harrell released on $100,000 bond earlier this week.
The 3 Doors Down bassist is hardly out of trouble, though, as he still faces charges of vehicular homicide by intoxication, unlawful possession of a controlled substance, violation of the implied consent law and possession of contraband.
On April 20, the band posted a brief statement to their website, writing, "We are deeply saddened by the passing of Paul Howard Shoulders, Jr. Our hearts and prayers go out to his family and friends at this difficult time." The band cancelled four planned shows in April and May.