Dr. Drew says doctors prescribing medication to Nick Reiner for his mental illness royally screwed up by changing his meds in the weeks leading up to his parents' murders ... but hindsight is 20/20, and there are all sorts of reasons why the meds get changed.

The good doc joined us on "TMZ Live" Thursday ... and we asked him about something we uncovered in our new documentary, "TMZ Investigates: The Reiner Murders: What Really Happened" ... Nick was stable on his meds for schizoaffective disorder a month before the killings, but then doctors decided to change things up.

Dr. Drew says meds get changed all the time when patients stress about how they're feeling or how their side effects are affecting them ... but that doesn't always make it the right move.

We're told Nick went into a spiral after his meds were changed ... and doctors did NOT attempt to place him under a psychiatric hold while they tried to adjust the meds and stabilize him, which may be key to his defense.

Dr. Drew says California makes it extremely hard for doctors to get their patients on a psychiatric hold ... breaking down the hurdles he and his colleagues face.

Catch the full interview on "TMZ Live" ... and watch out for our new doc, airing Friday night on FOX at 8 PM ET ... and dropping the next day on HULU.