Cocaine & Booze Nightmare: Inside Robin Williams' Downward Spiral Before Sobriety

His beloved fans saw him as a man full of laughter, but deep down, Robin Williams battled his own demons: alcohol and drugs, RadarOnline.com can reveal.

In an explosive new biography about the comedian titled Robin, writer David Itzkoff revealed dark secrets about Williams' downward spiral.

In the book, the comedian's friend, Dana Carvey, recalled just how present cocaine was on the set of Williams' first hit sitcom, Mork & Mindy.

Of course, Williams' career made it easier for the actor-comedian to get away with the heavy partying, the author suggested. Itzkoff claimed the actor's drug and alcohol abuse was "no secret," and that the white substance eventually "became an ingrained part of his nightly post-work routine."

"He was running like a mad man," Williams' Mork & Mindy co-star Pam Dawber is quoted in the book. "And of course drugs were playing into it. And I'd get mad at him and he'd deny it. It was just classic."

It was on the same set of Mork & Mindy where Williams was high on cocaine that he learned his best friend John Belushi died of a drug overdose. Itzkoff said Dawber broke the news to him and then immediately asked Williams to stop doing drugs himself.

"In a soft, solemn voice, he answered, 'That's never going to happen to me Dawbs,'" Dawber is quoted in the book.

From there, Williams cut cocaine and alcohol out of his daily routine cold-turkey. Of course, years later, in 2005, Itzkoff revealed his drinking became "bad again."

"Robin knew, too, that his drinking was doing real and lasting damage to his relationship with his family, the kind that would not be healed even if he got sober again and made amends for his wrongdoing," Itzkoff wrote.

Back then, Williams admitted it was "shameful."

As RadarOnline.com readers know, Williams was found hung dead in his home in 2014. It was revealed after his death that he suffered from Parkinson's Disease and brain-eating disease called Lewy Body Dementia.