Amanda Bynes Driving Under the Influence of ADDERALL ... Desperate Situation

Amanda Bynes was under the influence of Adderall when she was arrested for DUI ... sources familiar with the situation tell TMZ ... but her problems go much deeper than a criminal case.

We've learned the Adderall was prescribed by a doctor and she had taken the drug before getting stopped in Van Nuys Sunday and failing multiple field sobriety tests. She was arrested for DUI.

Adderall is a stimulant and we're told Amanda was "out of it" when she was stopped.

Fact is ... it's no defense to DUI that the drug in question was prescribed -- it's still operating a vehicle while impaired. But Amanda now faces a probation violation -- she's on probation for reckless driving -- and the fact that the drug was prescribed may help her there.

The deeper problem is this ... As TMZ first reported, Amanda has moved out of her parents house and is now living in Orange County. We're told she's smoking a lot of weed and is off her anti-psychotic meds for Schizophrenia and bipolarity.

Amanda's conservatorship ended earlier this month and we're told her parents saw no need to extend it because they believe she's better now, but everyone else in her life feels it was a horrible mistake to let the conservatorship lapse and they fear for Amanda.