Under mounting pressure from fellow politicians and others, his office released a statement last month from his unnamed doctor saying Jackson was receiving intensive medical treatment at a residential treatment facility for "a mood disorder."
The timing of the leave has raised questions, in part because Jackson is facing an ethics investigation in the U.S. House connected to imprisoned former Illinois Gov. Rod Blagojevich.
The House Ethics Committee is investigating allegations that Jackson was involved in discussions about raising money for Blagojevich's campaign in exchange for the then-governor appointing him to President Barack Obama's vacated U.S. Senate seat. Blagojevich is serving a 14-year prison sentence for corruption.