Butler, 42, entered the world-renowned Betty Ford Centre in California three weeks ago after reportedly "relying too much on prescription drugs" and in an effort to "head off a full-blown addiction".
Butler's problems were said to stem from his battle to recover from injuries he suffered while filming the Spartan epic, 300, in 2006.
According to the celebrity gossip news site TMZ.com, Butler began struggling with pain management, and the problem became exacerbated while filming his most recent project, Of Men and Mavericks, when he was badly hurt during a surfing accident shortly before Christmas last year.
TMZ said: "We're told Butler realised he was relying too much on prescription drugs and decided to head off a full-blown addiction by entering Betty Ford three weeks ago."
The site also alleges Butler "developed issues with cocaine, but the root problem appears to be the injuries and pain management".
Butler's representative Rupert Fowler told Radar Online last night: "Gerard has completed a successful course of treatment and has returned home in good health. We will not be making any further comment at this time."
Butler is not the first Hollywood star to have problems with prescription drugs. Last May, ex-Friends star Matthew Perry entered rehab for the third time in relation to a continuing problem with such drugs. He later met White House officials to discuss how to reduce such abuse in America.
Glasgow-born Butler has become one of Britain's best-known actors in Hollywood, having appeared in action films such as 300, Law Abiding Citizen and Ralph Fiennes's updated version of Coriolanus, as well as romantic films such as The Bounty Hunter, opposite Jennifer Aniston, and The Ugly Truth, opposite Katherine Heigl.
His CV also includes voice roles in DreamWorks's How to Train Your Dragon and its new sequel, currently in pre-production.
In a recent interview Butler, who originally trained as a lawyer, spoke frankly of the problems he endured in his mid-20s.
He said: "If I'd continued in the law, I don't think I'd be alive today. I was 27, I'd passed my degree and was working as a trainee solicitor, but I was heading down the wrong path and drinking far too much.
"The week before I was due to qualify, I got really wrecked at the Edinburgh Festival and was sacked. I now know that this was covering up the truth and that I was very unhappy with where I was headed."
In the same interview, he referred to the independence issue, saying: "I can't see any reason why Scotland shouldn't be independent. It's a different country with different attitudes, people and outlook, so why not?"
He also admitted: "For people in Britain, my success is a little baffling, like a conjuring trick. But, in all honesty, I worked my ass off to get where I am today. Hard work is something I believe in."
According to the Betty Ford Centre website, Dr Harry Haroutunian, physician director of the centre's licensed health professionals programme, took part last May in a White House conference call on prescription medication abuse.
Referring to an "epidemic" of prescription drug abuse in America, he said he had processed "hundreds of patients" through the centre and is "continually amazed at how many are completely unaware of prescription drug abuse in their own lives".
Some, he added, had taken such medications as Xanax and Valium with relative regularity for five, 10 or more years. "These drugs were never intended for that kind of use," he added.