Sources with direct knowledge tell us, Demi had been at a party in WeHo following a birthday celebration at Saddle Ranch on the Sunset Strip. After the drug dealer got the text, he came to her house and we're told the 2 of them freebased Oxycodone on tin foil.

Our sources say unbeknownst to Demi, the drug dealer had a bad habit of buying dirty drugs from Mexico ... and the batch in question appeared to be laced with fentanyl -- that's the same drug that killed Prince and Lil Peep.

We're told the dealer fled her home when he noticed she was in bed, breathing very heavily.

Our sources say Demi had been using the dealer since April.

TMZ broke the story ... someone on Demi's staff found her around 11:30 AM on the 24th. She had to be revived with Narcan.

Demi was hospitalized for 2 weeks and then went to an out-of-state rehab facility. Last Friday, she and her mom flew to Chicago for a few days where she received treatment from a renowned addiction specialist.