Nick Reiner‘s mental breakdown in the weeks leading up to the murders of his parents, Rob and Michele Reiner, is directly tied to a head-scratching decision by doctors to change the meds he was taking for his mental illness … sources with direct knowledge tell TMZ.

TMZ has a new documentary — “TMZ Investigates: The Reiner Murders: What Really Happened” — airing Friday at 8 PM ET on FOX. We explain in the doc, Nick was diagnosed around 2020 with schizoaffective disorder … a condition that manifests itself with schizophrenia symptoms — including delusions, hallucinations, and disorganized thinking — and mood disorder symptoms — including mania and severe depression.

Our sources say a month before the murders, Nick was stable on his meds for schizoaffective disorder … we’re told the meds were working just fine. For some reason, his doctors decided to change the meds. Our sources would not say why doctors made that decision … only that we would be shocked to learn the reason, which we’re told makes no sense.