Nick Reiner was diagnosed with schizophrenia, and in the weeks leading up to the murder of his parents, Rob and Michele Reiner, the meds he was taking made him “erratic and dangerous” … TMZ has learned.

Two sources with direct knowledge tell TMZ … Nick was under the care of a psychiatrist for mental illness. We’re told in the month before the murders, Nick’s behavior had gotten “alarming.”

He had recently received care at a Los Angeles-based rehab facility that specializes in mental illness and substance abuse. We’re told the facility charges $70,000 a month … and lots of rich, powerful parents send their troubled kids there.

Our sources say 3 to 4 weeks before the murders, doctors changed his meds and he became even more erratic. Our sources say doctors were trying to adjust the medications so Nick stabilized, but it wasn’t working.