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Here’s What Pot Really Does to You

A classic "Just Say No"-era anti-drug ad.  

What should a pot smoker be freaking out about in the long run? We’re still basically in the infancy stage of researching lifelong effects, but there is a lot of alarmism out there.

Like about what? A recent study trumpeted that pot “shrinks your brain”—comparing long-term users and nonusers, it found that a part of the brain (the orbito­frontal cortex) was smaller in smokers (it’s involved in processing reward signals and what’s called “executive function”). But there are so few serious longitudinal studies that it’s often hard to distinguish between association and causation, and in this case, causation may run the other way around: A previous study found that teenagers who were heavy smokers by 16 already had smaller orbito­frontal cortices at 12, before they started smoking (so it might be that small-brain-ness causes pot smoking, rather than vice versa).

Do we know anything for sure? The causality issue comes up in most of what we know about long-term effects—it’s damn hard to know whether something is caused by or causes marijuana use, whether it’s a smaller hippocampus (the brain region housing learning and memory), “functional connectivity” in areas governing alertness and memory, intelligence (as measured by IQ), or mental illness like depression and anxiety—conditions for which users may simply be self-medicating. But at the very least we can say that the list of conditions pot is associated with are … not the kinds of things you really want to be associated with.

On the panic scale, just how much should I worry about mental illness? The jury’s still out—it’s perhaps pot’s weightiest, most-studied question. In those with bipolar disorder, pot can seemingly trigger a manic episode, and hundreds of papers suggest a meaningful relationship with schizophrenia—not that pot causes it, exactly, but that it can exacerbate the condition and accelerate the onset (by as much as six years). Users are twice as likely as nonusers to develop a mental illness of some kind, but, again, the causality isn’t clear. The short answer is if you’re mentally healthy, don’t sweat a little burst of dysphoria; if you’ve got other issues, you might want to heed that paranoia and put down the pot.

Should I worry especially about my teenager getting high? Predictably, a 2012 paper suggesting teens can lower their IQs by up to eight points by smoking worked parents into a lather (Daily News headline: WHY THEY CALL IT “DOPE”), but newer studies have seriously undermined those findings. Still, even though the causality isn’t clear, there are a number of adverse associations to stress about: Teens who smoke pot do less well in school; drop out at higher rates; exhibit impaired memory, decision-making, and risk assessment; and earn less and commit more crimes later in life. Researchers have a schoolmarmish blanket term for this: “diminished lifetime achievement.” (In the 1960s, they called it “amotivational syndrome.”)

Why? It may just be that those kinds of kids are drawn to this drug. (Strangely enough, that recent study about brain shrinkage showed better connectivity in those smaller brains.) But it’s also the case that the brain keeps developing into a person’s 20s, and adolescents may alter the process by getting high. With heavy-enough use, neural connectivity may become impaired—fewer fibers in the hippocampus, for instance, or in the node for alertness. And lastly, THC has also been shown to make rat brains more sensitive to other drugs. No kidding—as far as the brain is concerned, it really is a gateway drug.