"Buffy the Vampire Slayer" star Nicholas Brendon says he made a bad decision to mix pain meds with alcohol this weekend ... and now he's going to rehab to get help.
Brendon posted an apology on Facebook Monday afternoon ... saying he was on pain meds for 5 days to manage a cyst behind his knee -- but when he drank as well, things got bad.
We broke the story -- cops say Brendon was causing a disturbance in the lobby of a hotel Friday night in Idaho -- and was arrested when he didn't cooperate with officers.
Brendon says he plans to check into a rehab facility, and adds ... "I apologize. Those two small words don’t begin to express my regrets, but they do mark the beginning of my recovery."