Watch "Dr. Drew" Friday night at 9 ET on HLN to hear about Dr. Drew's close relationship with Jeff Conaway.

Los Angeles (CNN) -- Actor Jeff Conaway, who was in the TV series "Taxi" and the movie "Grease," died Friday morning, his manager said.

While pneumonia was the cause of death, the doctor who treated him for drug addiction for years says it was his dependence on prescription painkillers that eventually cost him his life.

"Jeff was a severe, severe opiate addict with chronic pain, one of the most serious and dangerous combination of problems you could possibly interact with," Dr. Drew Pinsky said during a taping for Friday night's "Dr. Drew" on HLN.

"The pain seemed to be motivating him back to the opiates, and I told him for years that it was going to kill him," Pinsky said.

Conaway, 60, suffered from pneumonia and sepsis in recent weeks and had been in a medically-induced coma in an Encino, California, hospital for two weeks, manager Phil Brock said.

His family surrounded Conaway in his hospital room Thursday afternoon when he was taken off life support, Brock said.

"He was the consummate performer and entertainer," Brock said. "We're thankful his struggles are over, but we know he will be missed by legions of fans worldwide."

On hearing about his death, "Grease" co-star John Travolta said: "Jeff Conaway was a wonderful and decent man and we will miss him. My heartfelt thoughts are with his family and loved ones at this very difficult time."

Conaway's struggle with alcohol and drug addiction was chronicled in 2008 on the TV reality show "Celebrity Rehab With Dr. Drew."

"What happens is, like with most opiate addicts, eventually they take a little too much, not much more than usual, and they aspirate, so what's in their mouth gets into their lungs," Pinsky said. "That causes a rapidly progressing and overwhelming pneumonia that they usually don't know that they have, because they're sort of too out of it because of the drugs and by the time they get to the hospital it's too late. That's what happened with Jeff."

There was no evidence he ever intentionally overdosed, Pinsky said.