From Wikipedia.com:

Phoenix died at age 23 from a drug overdose of heroin and cocaine (speedball) outside the Viper Room, a Hollywood night club partly owned by actor Johnny Depp. At the time of his death, Phoenix was acting in the starring role of the movie Dark Blood, which was never completed. He was cremated in Gainesville, Florida a few days later.

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Same thing that killed John Belushi and Chris Farley.