As the Mail on Sunday has reported, filmmaker Furnish sought alcohol to help cope with his partner's worldwide fame.
John notes in his autobiography, Me, that he learned of Furnish's drinking problem after finding empty wine bottles all over their various homes around the world.
He also found his husband keeled over the front of his computer.
“I always thought David had slipped into Elton John World with remarkable ease and confidence, but it turned out that a lot of things I was completely used to living with, that I just saw as a fact of life, made him completely anxious,” John wrote.
“He didn’t like being photographed all the time, or being under press scrutiny, or public speaking at Elton John AIDS Foundation events.”
The advertising executive also struggled with loneliness caused by John's frequent touring.
John and Furnish entered a civil partnership in 2005 and the couple exchanged their wedding vows in 2014 when gay marriage was legalized in England.
At one point, four years after they'd become parents to their adopted sons Elijah and Zachary, John believed the producer's booze addiction would lead to the downfall of their relationship.
“It all came to a head in Los Angeles in 2014, two days before I was due to start a US tour… David was feeling low and wanted me to stay the extra night with him. I said no. We had a huge row, I went anyway,” John wrote.
“The next morning, David called and we had a row that made the previous day’s row look like a light-hearted disagreement over what to have for lunch: the kind of argument where you come off the phone teary and reeling, where things are said that make you wonder whether the next time you communicate it’ll be through lawyers.”
The next time John spoke to Furnish, 56, he had checked himself into a clinic in Malibu, California.
Upon his release, the 72-year-old star, who met Furnish three years after his own battle with alcohol, accompanied his partner to Alcoholics Anonymous meetings.
Readers know John has shared some juicy information in his tell-all memoir.
Radar previously reported that the "Don't Let the Sun Go Down on Me" singer shared some gritty details about his years-long battle with cocaine.
In the book, John shared a goodbye letter he wrote to the drug he referred to as the "white lady."
“You never cease to amaze me – I’ve sent cars to pick you up and I even sent planes so that you and I could spend some hours or days together,” he wrote of the lengths he went to get cocaine during his 16-year addiction.
The star admitted his addiction caused him to feel “tired and hateful” of himself and declared he was “prepared to accept humility.”