Brooke Mueller says she's a "stage 4" drug addict and it's only a matter of time before she relapses again.

Charlie Sheen's ex-wife opens up about her struggles with substance abuse in a new interview with People, telling the outlet she's stuck in a cycle where she gets sober and relapses.

Brooke says ... "When we're at stage four, it typically means that cycle where you just have these relapses, no matter how well you get, and no matter how many years, it's just a matter of time."

She says she is being honest about addiction issues because "if I try to sugarcoat it, I'd be lying to myself."

Brooke says every so often she convinces herself she needs prescription drugs ... and then she goes through a bottle of pills in 36 hours.

She says Charlie is always her first call when she needs help for her addiction and says he's better at avoiding vices than she is.

As for her two kids with Charlie, Brooke says they're doing well but have been through a lot ... adding, "The sad reality, is they get used to it -- Mommy is going to relapse again one day."