"I have a liver disease," Cassidy told producer Saralena Weinfield, according to People. "There is no sign of me having dementia at this stage of my life. It was complete alcohol poisoning."
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Cassidy also told Weinfield that he had not been honest about his health.
"The fact is that I lied about my drinking," he said. "I did this to myself to cover up the sadness and the emptiness."
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Cassidy, best known as the teen heartthrob from the 1970s show The Partridge Family, died in November of last year.
The singer had struggled with alcohol abuse in the later part of his life, with three DUI arrests in five years, according to People. He told loved ones he had stopped drinking in 2014 and he later announced he had dementia.
The documentary, which airs on Monday, followed Cassidy in the recording studio as he attempted to stage a comeback amid health problems. Cassidy called producers after becoming ill in the recording studio and having to be taken to the hospital. That's when he told them he had never had dementia.
The producers were shocked and debated what to do about the documentary, they told People, but ultimately decided to share his story.
"I think it will strike a chord with people," producer John Marks told People. "He wanted to share this very private part of his life, and to be honest once and for all. And I think he succeeded in doing that."