Lindsay Lohan's Adderall dependence -- the result of a medical misdiagnosis -- may have been the reason she went off the rails.
As we first reported, the docs at the UCLA rehab facility believe Lindsay was misdiagnosed with Attention Deficit Hyperactivity Disorder ... and was prescribed Adderall to treat the phantom affliction.
Dr. Joe Haraszti -- a prominent L.A. addiction specialist -- tells TMZ ... people who take Adderall when they don't need it can experience similar effects as people who use cocaine or methamphetamine.
Dr. Haraszti tells us ... people who unnecessarily take Adderall can display manic symptoms ... and often do things like "driving around until all hours of the morning ... smoking heavily ... tweeting ... and texting all night long." The doc adds, "They can become very impulsive."
The doc also notes that people in that situation "might then complain of insomnia and then take Ambien or other sleep aids to help fall asleep ... it's a vicious cycle"
Dr. Marc Kern -- another addiction expert -- tells us alcohol abuse is also very common among patients who take Adderall.
Dr. Kern tells us, "The Adderall counteracts the sedative side effects of alcohol use … making these people often drink more than someone who does not take the drug."
Bottom line -- a bad medical diagnosis may have set the ball rolling that unhinged Lindsay's life.