The 'Reefer Madness' notion that smoking pot could turn you into a degenerate, criminally insane psychopath has long been a joke.

No reliable research supports the idea that cannabis — the plant popularly known as marijuana — can harm your mental health, according to Dost Öngür, an associate professor of psychiatry at Harvard Med School. But there's one specific, notable exception, he says: cannabis use during early teenage years appears to be a risk factor for later developing psychotic disorders like schizophrenia.

"It's really important to be clearheaded about this. This discussion about cannabis and psychosis is not about 'drugs are bad' or 'pot will hurt you or kill you,'" Öngür explains. "It's a very specific medical risk: In teenagers — and, in particular, in the 12-15 age range — teenagers who are smoking daily are at about threefold higher risk of developing schizophrenia down the road."

The risk being tripled is small — from a roughly one percent chance of developing schizophrenia to a three percent chance. But spread out over populations, that could mean thousands of people becoming psychotic who might not have if they had avoided pot as teenagers.

"So we're not talking about smoking pot and having a reaction to it," Öngür says. "We're talking about pot smoking that seems to be modifying the brain and then years later actually manifesting as a diagnosis of schizophrenia."

What accounts for the connection between early marijuana use and schizophrenia? We don't know, Öngür says, but we can speculate based on what we know pot does in the brain.

"Cannabis, of course, contains chemicals that interact with brain receptors. Those receptors are called endocannabinoid receptors. Endocannabinoids are chemicals in the brain that are involved in regulating brain activity. So smoking cannabis is probably modulating those receptors in a way that's causing permanent changes that aren't good for you," Öngür explains.

The new Massachusetts law legalizing marijuana restricts use to adults over the age of 21. But Öngür worries that increased availability among adults will mean more kids will be able to get their hands on the drug.

"I think the most clear-eyed advice we can give kids who are entering [their] teenage years is, it's not healthy for you to be smoking pot now," he warns. "As an adult, if it's legal, if it's in a different context, that's something you can consider separately. But smoking cannabis in [the] teen years, specifically [the] 12-15 year age window, is asking for trouble."

Because different brains develop at different speeds and in different contexts, Öngür says, individuals could experience adverse effects in their twenties. Not only does he support the proposal to raise the legal age for pot to 25 — when it comes to his own students and his kids, he says the safest course is to avoid pot until you're 30.