Los Angeles County coroner Craig Harvey says coroners did not conduct a toxicology test because Conaway had been hospitalized for weeks prior to his death, so his blood would have come back clean.
However, Harvey says when Conaway entered the hospital May 10, opiates and other drugs were in his system.
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.
Conaway was known for playing vain, struggling actor Bobby Wheeler on "Taxi" and Kenicke, the swaggering best friend of John Travolta in "Grease."