The 38-year-old star reportedly collapsed at his home in Oakwood in the early hours of Wednesday morning (10Mar10) and his distraught mother called emergency services. He was taken to St Joseph's hospital in Burbank, where he was pronounced dead.
Sergeant Frank Albarren, a spokesperson for the Los Angeles Police Department, tells MSNBC.com, "It was an apparent overdose. Unknown what type of medication."
Haim had reportedly been sick in the days before his death, suffering flu-like symptoms, according to TMZ.com.
The actor had a history of drug problems and had spoken candidly of his longrunning battle with substance abuse.
Born in Ontario, Canada, Haim broke into acting in the 1980s on Canadian TV show The Edison Twins, before making his film debut opposite Sarah Jessica Parker and Robert Downey Jr. in 1984's Firstborn.
He appeared opposite Point Break actor Gary Busey as a paraplegic boy in the big screen version of Stephen King's Silver Bullet, before shooting to international stardom as the lead in 1986's Lucas - alongside Charlie Sheen and Winona Ryder.
Haim went on to film eight hit movies with his child star namesake Corey Feldman, including The Lost Boys, License to Drive, Watchers and Dream A Little Dream, leading the actors to be dubbed 'The Two Coreys'.
Haim was awarded two Young Artist Awards during the 1980s - Exceptional Young Actor in TV show A Time to Live in 1985, and Best Young Actor in 1989, which he shared with Feldman for their parts in License to Drive.
But as rumours of a serious drug problem began to surface in the 1990s, his career took a dip with a string of straight-to-video movies including Blown Away and National Lampoon's Last Resort.
In 1993, he was charged for pulling out a fake handgun during a bust-up with his business manager. The charges were later reduced from felony to misdemeanour.
In 1996 he was sued for $375,000 (£250,000) after pulling out of film Paradise Bar because of his drug problem, which he had reportedly failed to mention on his insurance form. With his financial problems spiralling out of control, Haim filed for bankruptcy in 1997.
The actor reportedly checked himself into rehab 15 times in a bid to battle his drug demons, and in 2001 was rushed to the UCLA Medical Center in Santa Monica, California after reportedly suffering a drug-induced stroke.
But in 2004 he moved back to his native Toronto and appeared to have overcome his problems.
In recent years, Haim attempted to resurrect his career, filming reality show The Two Coreys with Feldman in 2007, and in 2008 he shot minor scenes for Lost Boys: The Tribe, reprising his 1987 role.
The two actors' screen reunion was short-lived - Feldman, who has also struggled with drug problems, reportedly refused to continue working and communicating with Haim until he beat his addiction.
At the time of his death, Haim had signed up to appear in a number of productions, which were slated for release later this year (10), including The Science of Cool opposite Mischa Barton, The Throwaways alongside pop star-turned-actor Luke Goss, and action thriller The Dead Sea.
Haim was briefly engaged to Nicole Eggert, Holly Fields, and Cindi Guyer, and has previously dated Victoria Beckham and Alyssa Milano, but it is unknown if he was in a relationship at the time of his death.