Demerol is as addictive as morphine and levels can store up in the body and cause such serious effects as seizures and tremors. When used with alcohol, other opioids or drugs that induce central nervous system depression, it can be particularly dangerous.
Demerol interacts dangerously with "about a hundred" other drugs, according to Dr. Christopher Cannon, a cardiologist at Brigham and Women's Hospital in Boston.
"Like any narcotic, people build up tolerance to opiates," he told ABCNews.com. "They need higher doses to get the pain effects. But to be on IV Demerol is practically unheard of from someone who hasn't had surgery. It's very unusual."
With Demerol, "there is a whole laundry list of things that are counterindicated, to be avoided or caution is advised," said Cannon.
"Once you start taking additional medications, six, seven or eight at a time, interactions start happening and it's a set up for this -- the feared complication -- that's why we do EKGs on various medications that are known to affect the heart."
Plastic surgery aside, looking over Jackson's physical decline over the past decade, drug use certainly make sense, according to Cannon.
"Emaciation is a classic thing [in drug addiction] and he fits that in his face," said Cannon. "It's very possible that it's part of his physical change and decline. Certainly mentally -- he has been very erratic in his behavior. "
During the 2005 child molestation trial, the judge ordered prosecutors to return hundreds of items that were not deemed "contraband," according to The Associated Press at the time.
Those items included syringes, the drug Demerol and prescriptions for various drugs, mainly antibiotics, that were in different people's names.
Prior to Jackson's arrest and the raid on his Neverland ranch, some of the star's doctors talked about staging an intervention, according to Schaffel.
"We actually didn't do it, but we were talking about it when we were getting ready to leave on a six month trip," he said. "We had spent Christmas and New Year's at Elizabeth Taylor's place in the Swiss Alps and we were headed to do an award show in German and France and South America."
"The misconception is talking about people enabling him," said Schaffel. "About 90 percent of the people had nothing to do with the drug use. His regular doctors didn't give him drugs."
Schaffel said he had last seen Jackson a few years ago, when a British television network approached the producer about running the private home movies.
"When you've got that kind of money, you can get your drugs," he said of Jackson. "We call them Dr. Feel Goods. We tried to intercept and block them, but when you have that kind of money, you can get them on your own."
ABC News' Karin Halperin contributed additional reporting for this story.