Nick Reiner‘s weight gain set him on a murderous path, but it didn’t have to be that way … just ask Dr. Drew.

The good doc joined us on “TMZ Live” Monday and told us Nick likely would have benefited from being prescribed weight-loss drugs to counteract a common side effect of the anti-psychotic drugs he was taking.

As we uncovered in our new documentary, “The Reiner Murders: What Really Happened,” Nick’s unravelling that resulted in the murders of his parents, Rob and Michele Reiner, is directly connected to his gaining weight.

Nick was stable on a cocktail of drugs until a month before the murders, when he complained to doctors about weight gain. His meds were changed, and sources say the new mix sent him into an erratic, dangerous state. We’re told he had a complete break from reality at the time of the murders.

Dr. Drew says psychiatrists, particularly younger ones, are now prescribing patients GLP-1 drugs — like Ozempic and Mounjaro — as a way to counterbalance side effects from the other pills … though he says it’s a delicate balance.

In Nick’s case, Dr. Drew says GLP-1 drugs may have helped … and who knows where Nick, Rob, and Michele would be if the meds were never changed.