Conaway died May 10 after being in a coma for more than two weeks.
The actor had a long history of substance abuse, but toxicology tests were not performed on him. Conaway struggled publicly with his addictions on "Celebrity Rehab with Dr. Drew."
Conaway, who had repeated back surgeries for an injury, blamed his drug abuse, in part, on his lingering back problem.
The Los Angeles County Coroner's Office said his blood would have come back clean because he'd been hospitalized for weeks prior to his death.
Conaway was known for playing vain, struggling actor Bobby Wheeler on "Taxi" and Kenicke, the swaggering best friend of John Travolta in "Grease."
The Associated Press contributed to this report.