Lindsay Lohan: NO Adderall, NO Rehab!

Lindsay Lohan may leave rehab as fast as she arrives, if they dare try and take her precious Adderall away ... and we've learned that's exactly what the facility plans to do.

TMZ broke the story ... Lindsay will do her 90 days of rehab at the Seafield Center in Westhampton Beach, NY. An official source at the Center tells TMZ although they allow people to take drugs prescribed by a doctor, they are suspicious of Adderall because of its many abuses -- lots of people say they take it for ADHD, when they're really using it for weight loss and getting high.

We're told Seafield tries to ween patients off Adderall and give them other, less addictive meds for ADHD.

As for Lindsay -- who's been a hard-core Adderall user for years -- she's telling friends she absolutely refuses to take an Adderall substitute. She says she's tried other meds and Adderall is the only Rx that works for her.