Haim died at 2:15 a.m. Wednesday at Providence St. Joseph Medical Center in Burbank, Los Angeles County coroner's Lt. Cheryl MacWillie said.
An autopsy will determine the cause of death and there were no other details, she said. Police Sgt. Michael Kammert said there's no evidence of foul play.
Haim had flulike symptoms before he died and was getting over-the-counter and prescription medications, Police Sgt. William Mann said. The cause of death is unknown, Mann said.
"He could have succumbed to whatever (illness) he had or it could have been drugs. Who knows?" 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 enormous complex is known as Oakwood and is popular with young actors, Kammert said.
Haim acknowledged his struggle with drug abuse to The Sun in 2004.
"I was working on Lost Boys when I smoked my first joint," he told the British tabloid.
"I did cocaine for about a year and a half, then it led to crack," he said.
Haim said he went into rehabilitation and was put on prescription drugs. He took both stimulants and sedatives such as Valium.
"I started on the downers which were a hell of a lot better than the uppers because I was a nervous wreck," he said. "But one led to two, two led to four, four led to eight, until at the end it was about 85 a day."
In 2007, he told ABC's "Nightline" that drugs hurt his career.
"I feel like with myself I ruined myself to the point where I wasn't functional enough to work for anybody, even myself. I wasn't working," he said.
The Toronto-born actor got his start in television commercials at 10 and earned 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 and he became a teen heartthrob with his roles in the 1986 movie "Lucas," and "The Lost Boys," 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.
But in 1997 he 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.
In a 2007 interview on CNN's "Larry King Live," Haim called himself "a chronic relapser for the rest of my life."