Fergie was "hallucinating on a daily basis" during her battle with drugs.
The 'Big Girls Don't Cry' hitmaker admits she was suffering from a "chemically induced psychosis" when she used to take crystal meth.
She said: "I was [suffering from] chemically induced psychosis and dementia. I was hallucinating on a daily basis. It took a year after getting off that drug for the chemicals in my brain to settle so that I stopped seeing things. I'd just be sitting there, seeing a random bee or bunny."
And the 42-year-old singer - who is now clean of the substances - admits at one point that she thought she was being tracked by the CIA and FBI.
Speaking about how she was convinced that "the CIA, FBI and a SWAT team" were tracking her, so she tried to hide at a church, she added: "They tried to kick me out, because I was moving down the aisles in this crazy way, as I thought there was an infrared camera in the church trying to check for my body. I bolted past the altar into a hallway and two people were chasing me. I remember thinking, 'If I walk outside, and the SWAT team's out there, I was right all along. But if they're not out there, then it's the drugs making me see things and I'm going to end up in an institution. And if it really is the drugs, I don't want to live my life like this any more, anyway.' I walked out of the church; obviously there was no SWAT team, it was just me in a parking lot. It was a freeing moment."
Fergie - who has son Axl, four, with her estranged husband Josh Duhamel - admits there was a time where she found drugs "a hell of a lot of fun" but says that soon ended.
She told iNews: "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."