Brooke Mueller -- I Was Abusing ADDERALL During Jewelry Bender

Brooke Mueller is coming clean to her closest friends ... admitting she WAS abusing prescription medication before she was rushed to the hospital last week ... TMZ has learned.

Mueller has been telling her friends ... she has a valid prescription for Adderall to treat her attention deficit hyperactivity disorder ... but admits she "fell off the wagon" last week and took way more than her prescribed dosage.

We broke the story ... Mueller's assistant found Brooke passed out cold on Friday morning and called 911, fearing Brooke had overdosed ... which was totally plausible considering Brooke's well-documented struggle with substance abuse.

Hours after Brooke was released from the hospital, she told TMZ she hadn't been abusing "illegal drugs" (including cocaine) ... but now, we're told she's finally owning up to the truth.

According to reports, Adderall is one of the most addictive and abused prescription drugs in America -- mainly because of its cocaine-like effects.

Our sources tell us, Brooke -- who's now in the jewelry business -- took the pills because she was desperate to meet a deadline for a jewelry order ... and didn't think she could afford to waste time sleeping.