As per the terms of her probation, failing means an automatic 30 days in jail.
Sources close to Lindsay say that she insists it's not true, but TMZ is adamant in confirming it is indeed true.
D.A. spokesperson Sandi Gibbons says her office has not been "officially notified" of the test results and no court date has been set yet.
One source connected with Lindsay tells TMZ, "People with addiction problems don't get better magically, and she's really been making an effort to make herself better."
If this is true, we have no words. We can’t believe she would chance going back to jail just to get high.
UPDATE: According to TMZ, Lindsay tested positive for cocaine!
UPDATE #2: Lindsay has responded to the allegations, telling Us Weekly, "I'm fine. They're all nuts."
Her lawyer Ed MacPherson added, "I have no idea at this point whether or not there is any merit to this."
L.A. County Sheriff's Department spokesperson Steve Whitmore says, "I haven't been officially notified by anyone" about a failed Lohan drug test or the star's return to prison.
Kathy Roberts of L.A. Superior court also said, "nothing has been filed or anything entered into the court calendar. We can't confirm [the report]."
A source has also come forward to say Lindsay was spotted at NYC club Boom Boom Room until really late on September 11. A witness tells the mag, "It looked like she didn't want to be noticed."