The 3rd Rock from the Sun star, 44, has opened up about her life-long struggles in her upcoming memoir Guts: The Endless Follies and Tiny Triumphs of a Giant Disaster, in which she tells all about trying to quietly battle her demons on her own.
But Johnston had no choice but to turn to professionals to help her face up to substance abuse when her penchant for alcohol caused a stomach ulcer, the result of heavy drinking, to explode - leaving her in immense pain in 2006.
She says, "I drank my whole life, like, since I was in high school, because I'm from Wisconsin and that's what you do. And then eventually it got to the point where, if you were like, 'Look, I have this pill, (that) I found on the subway', and it was covered in blood, I was like, 'I'll try it'...
"I had a very terrifying incident. Remember when I was scary skinny, about five years ago?... I was 'anorexic', I was 'desperately losing weight so I could have a career again', but actually what had happened was that my stomach had burst open, which is what happens when you take a lot of Vicodin (strong painkillers), a lot, everyday, with two bottles of wine a night, for years... So that's what's the book about. But it's also very honest, because I think sometimes people skate over (things)."
And although Johnston finds humour in her former health scares, the actress reveals she's finally on the straight and narrow following a stint at the Boxholm, Iowa rehabilitation centre.
She adds, "I was in Boxholm... Addicts have different brains. I think it's the wiring and the chemistry. I do say no (to drugs and alcohol now) - I've been clean and sober for five-and-a-half years."
Johnston's book is due out in March (12).