Lindsay Lohan Fails Drug Test

9/17/2010 5:37 PM PDT

Lindsay Lohan Drug Test Results -- Failed!

Lindsay Lohan has failed a drug test in her criminal case ... TMZ has learned.

We're told the court-mandated drug test in question occurred last week.  According to the terms of Lindsay's probation, she will get 30 days in jail for a positive drug test.

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."

And D.A. spokesperson Sandi Gibbons tells TMZ her office has not been "officially notified" of the test results and no court date has been set yet.

UPDATE:  Sources close to Lindsay tell us she is saying it's not true -- insisting she has not failed, but we have confirmed it is indeed true.