Medical science has known for a long time that too much marijuana, especially smoked in adolescence, can make people crazy.

Now, as President Donald Trump mulls whether to loosen federal controls of pot, we have new evidence that marijuana-induced psychosis can create unimaginable horrors.

The murderer who gunned down two children and injured 18 others at a Minneapolis Catholic school last week blamed pot in part for his killing spree, The Post has reported.

“Gender and weed f–ked up my head,” 23-year-old shooter Robin Westman claimed.

“I wish I never tried experimenting with either. Don’t let your kids smoke weed or change gender until they are like 17,” Westman wrote in a diary entry made public after the carnage.

The killer not only reportedly smoked marijuana, but also worked at a cannabis dispensary “for several months earlier this year,” according to reports.

We can’t say for certain that the shooter’s marijuana use contributed to his rampage — though he clearly thought it did.

But the relationship between marijuana, psychosis and schizophrenia is well established and hardly up for debate.

According to a 2019 review of systematic reviews, cannabis users are more likely to develop psychotic illnesses.

Greater cannabis use elevates that risk, the analysis found, and those who use cannabis develop psychosis earlier than those who don’t.

“Cannabis use was also associated with increased relapse rates, more hospitalizations and pronounced positive symptoms in psychotic patients,” the review notes.

Adolescent use of cannabis is particularly risky: Another review, published in 2021, found that adolescent use is associated with a 71% increase in risk for psychosis and predicts an earlier onset.

Advocates of legalization, of course, will protest that correlation is not causation.

They’ll insist that people at risk for serious mental illness may use cannabis to “self-medicate.”

Yet analyses which use the random variation of genes — so-called Mendelian randomization studies — allow for isolation of the effects of cannabis use on schizophrenia.

One such study, published in 2017, indicated that cannabis use increases the risk of schizophrenia by an estimated 37%.

Of course, not everyone who is violent is schizophrenic, and most people who are psychotic are not necessarily violent.

But one needs only to look at both the Minneapolis shooter’s words and his actions to conclude that he was seriously unwell — and that that insanity contributed to his evil spree.

Maybe, but for the pot, his victims would be alive today.

Why bring all this up? Because Trump is reportedly on the verge of loosening federal controls on marijuana.

This would not, as I and others have argued, lead to increased medical research on the plant.

Despite the lies of the industry, shifting cannabis from Schedule I to Schedule III in the federal government’s drug classification system would have little impact on scholars and scientists.

Instead, rescheduling pot would make it legal for state-legal marijuana businesses to deduct their business expenses — something they can’t do as long as marijuana is a Schedule I drug.

That’s why Big Weed has been clamoring for rescheduling. They’re the ones who will gain.

But that also means the legal marijuana industry will get bigger — including businesses like the one the Minneapolis shooter worked at.

That means more people smoking pot, more people becoming addicted to pot and more adolescents using heavily — some of whom will, inevitably, be driven crazy by their use.

The president, then, should look to Minneapolis before he makes any decisions on marijuana’s legal status.

Surely there are big corporate donors clamoring for him to hand them the cash bonanza rescheduling would mean.

Against them, he needs to weigh the children whose lives were lost in Minneapolis — and the possibility that more legal weed will mean more of the same.

Charles Fain Lehman is a fellow at the Manhattan Institute and senior editor of City Journal.