The Black Eyed Peas singer forced herself to stop taking drugsafter going to church one day when she was "really paranoid" the FBI (Federal Bureau of Investigation) and the SWAT (Special Weapons And Tactics) team were following her, but speaking to the Lord made her see sense.
She said: "My brain had been playing a lot of tricks on me. You`re kind of living in this alternate reality.
"I started getting really paranoid. So I went one day into this church and I thought that the FBI and the SWAT teams were outside the church ... so I had a conversationwith God ... and I said, `Alright, if I go out there and the FBI and the SWAT team`s not out there, then it`s the drugs and I`m stopping.`
"I went outside of the church ... and there was no SWAT team, there was no FBI, just me and God. And I kept my promise. That day. That was it."
Fergie admits she started taking hard drugs as a teen when she felt unable to tell her fellow pop group members in Wild Orchid she wanted to launch a solo career.
She explained: "What I should have done was said, `Girls, it`s really time for me to go [out] on my own. I need to fulfil this dream of mine to have a solo album.` And I didn`t know how to do that, I didn`t know how to deal with that confrontation ... So I started going out and taking Ecstasy and from Ecstasy I went to crystal meth.
"At first, with any drug, it`s great in the beginning, then slowly your life starts to spiral down. [I was] 90 pounds at one point."
As well as drugs, the 37-year-old singer - who has been married to actor Josh Duhamel since 2009 - also experimented with women and admits she had a "bisexual or sexually open free spirit" during her college years.
Speaking to Oprah Winfrey on her chat show, `Oprah`s Next Chapter`, the singer added: "I never dated women. It was more fun. I think that a lot of women experience this sort of thing in college. Tour was like college in a way. I mean I guess I would call it bisexual or sexually open, free spirit ... what have you."