Lindsay Lohan drew a line in the sand during plea negotiations in her lying-to-cops case -- she'd agree to rehab ONLY if the facility would allow her to continue taking Adderall ... sources close to LiLo tell TMZ.

Our sources say Lindsay has taken Adderall -- used to treat Attention-deficit/hyperactivity disorder and misused for various other purposes -- daily for years.  She believes she needs it to function.

We're told Lindsay was steadfast -- either attach Adderall to the deal or she'd take her chances at trial.  Sources say Lindsay's lawyer, Mark Heller, assured her he could find a lockdown rehab facility that would accommodate her demand.

Heller may not be able to deliver ... fact is, there's no such thing as a lockdown rehab facility that would take Lindsay, much less allow her to take the drug.

It's pretty ironic ... when Lindsay was jailed back in 2010, she got a doctor to prescribe Adderall while she was behind bars ... even though her ADHD diagnosis was called into serious question.

A number of people in Lindsay's life believe it was the Adderall that led to her downfall.