Haim died at the age of 38 early Wednesday at Providence St. Joseph Medical Center in Burbank, Los Angeles County coroner's Lt. Cheryl MacWillie said.
Haim acknowledged his struggle with drug abuse to a British tabloid in 2004.
"I was working on 'Lost Boys' when I smoked my first joint," he told The Sun. "I did cocaine for about a year and a half, then it led to crack."
Haim said he went into rehabilitation and was put on prescription drugs. He took stimulants and sedatives.
"I started on the downers, which were a hell of a lot better than the uppers because I was a nervous wreck," he said.In a 2007 interview on CNN's "Larry King Live," Haim called himself "a chronic relapser for the rest of my life." In 2007, he told ABC's "Nightline" that drugs hurt his career. "I wasn't functional enough to work for anybody, even myself. I wasn't working," he said.
An autopsy will determine Haim's cause of death. There was no evidence of foul play, police Sgt. Michael Kammert said.
"As he got out of bed, he felt a little weak and went down to the floor on his knees," Assistant Chief Coroner Ed Winter said. His mother called paramedics.
Haim, who gained attention for his roles in "Lucas" and "The Lost Boys," had flulike symptoms before he died and was getting over-the-counter and prescription medications, police Sgt. William Mann said.
"He could have succumbed to whatever (illness) he had or it could have been drugs," Mann said. "He has had a drug problem in the past."
Haim was taken by ambulance to the hospital from an apartment in Los Angeles near Burbank.
The Toronto-born actor got his start in television commercials at 10 and developed a good reputation for his work in such films as 1985's "Murphy's Romance" and his portrayal of Liza Minelli's dying son in the 1985 television film "A Time to Live."
His career peaked when he became a heartthrob with his roles in the 1986 movie "Lucas" and "The Lost Boys" in 1987 in which he battled vampires.
In later years, he made a few TV appearances and had several direct-to-video movies. He also had a handful of recent movies that have not yet been released.
In 1997, Haim filed for Chapter 11 bankruptcy protection, listing debts for medical expenses and more than $200,000 in state and federal taxes.
His assets included a few thousand dollars in cash, clothing and royalty rights.
In recent years, he appeared in the A&E reality TV show "The Two Coreys" with his friend Corey Feldman. It was canceled in 2008 after two seasons. Feldman later said Haim's drug abuse strained their working and personal relationships.