The Black Eyed Peas star began taking drugs as a teenager and grew dependant on crystal meth.
And 42 year old Fergie has now revealed that her addiction was so bad, that she was hallucinating every day.
Speaking to i Newspaper: “At my lowest point, I was [suffering from] chemically-induced psychosis and dementia.
“I was basically hallucinating on a daily basis. It took a year after getting off that particular drug for the chemicals in my brain to settle so that I actually stopped seeing things.
“I’d just be sitting there, seeing a random bee or a bunny.
“The drugs thing, it was a hell of a lot of fun… until it wasn’t. But you know what, I thank the day it happened to me. Because that’s my strength, my faith, my hope for something better.”
Fergie and Josh plan to adopt
In the same interview, Fergie also opened up about her split with Transformers actor Josh Duhamel after eight years of marriage.
She said: “I’ve just released my first solo album in 11 years, and I’m out promoting it. So there’s really a lot going on.
“That’s why we wanted to find our footing with our separation before we announced it publicly: to make sure we really got it together for our kid.
“Thank the Lord for FaceTime! Because both Josh and I are working full-time right now. If you’re a full-time working parent, you’ll know what we’re experiencing.
“It gets tough and we get tired, but we’re trying our hardest to make sure that kid feels nothing around him but love.”