Lindsay Lohan -- Her Adderall Is On the Chopping Block at Betty Ford

Lindsay Lohan is NOT drug free at The Betty Ford Center ... TMZ has learned.

Sources connected with the rehab queen tell TMZ ... Betty Ford is allowing her to take the powerful drug, Adderall. The reason -- she has an Rx and says she's been diagnosed with Attention Deficit Hyperactivity Disorder.

Lindsay has made it clear over the last month -- she would only go to rehab if she's allowed to take the med. BUT ... sources tell us doctors at Betty Ford are currently evaluating Lindsay's diagnosis and the efficacy of the drug. If doctors disbelieve her diagnosis or the usefulness of Adderall, they will cut her off.

Adderall is commonly misused by lots of people -- especially actresses -- who take the drug to suppress their appetite.