The guy has been on various drugs since before Grease! I remember reading that he had lots of support from his Taxi cast members, but to no avail. The guy just was not happy if he was not fucked up on something.
In his later years Peter Lawford was an alchy to the degree that his Dr. told him, after one binge that landed him in the Hospital with a terribly destroyed liver, that his next drink would be his last. He stayed off the sauce for about a week, started back guzzling and sure enuff was dead a week later. His daughter was there to see it all. She said in her Bio that he knew it would kill him, but would rather die drunk than live sober.
Robert Downey Jr. was a stone addict that beat it after years of bouncing in and out of jail and rehab. As futile as the system is at curbing victimless crime, apparently Downey hated being cornholed in jail! Too bad Belushi, Farley, Joplin and all the rest lived in the pre millennium age where a cop would just give a stoned out celeb a ride home and not jail them.
