The other day I was walking down the street with my youngest child, Kobi, enjoying an early spring day in New York.
The air was chilly but the sun was out. People were optimistically clad in shorts and T-shirts, as if dressing prematurely for warm weather would somehow bring it on.
Even the rats, having become brazen enough lately to stroll around in full daylight, seemed to have a spring in their step.
At one point Kobi angled his head, sniffed the air, and turned to me congenially. “You smell that? That’s weed.” he informed me.
Indeed it was, and it seems like it’s everywhere.
Most Americans believe marijuana should be legal, an increasing number use it, and state legislatures are following along. California was the first to legalize medical marijuana back in 1996. By now, 37 states allow for medical use while 18 (and counting) permit recreational use.
On April 30, 2021, Cuomo legalized cannabis for recreational use in New York, and in no time at all its distinctive aroma became a pervasive and ubiquitous backdrop to the city.
Although it is still illegal for anyone under the age of 21 to possess, sell, or use any amount of marijuana, there is little doubt that legalization has normalized weed for young people everywhere.
According to one of my older kids, it’s gotten to the point that smoking weed on a school night - while it may raise eyebrows - is not seen by peers as a “problem” the way drinking on a school night might be.
But what if it is a problem?
Let’s look at it from a risk-benefit perspective, and focus on recreational marijuana which is how most young people use it.
The main benefit of marijuana is that, well, it feels good.
THC, the active component in Cannabis, has a similar structure to the naturally occurring neurotransmitter, Anandamide. Both chemicals work by attaching to cannabinoid receptors throughout the brain, including in the mesolimbic, dopaminergic reward system.
This is the same area of the brain that releases dopamine in response to other psychoactive substances and activities ranging from cocaine, to sex, to Instagram.
The main risk of marijuana is its potential effects on mental health.
Acute effects can include impaired memory, attention, judgment and cognition, and less commonly but more concerningly, paranoia, panic attacks and psychosis.
Long-term effects can include impaired learning and coordination, impaired sleep, the potential for addiction, and perhaps even an increased risk of schizophrenia.
If you think that this sounds alarmist for a substance that so many have used to no ill effect, I agree. Maybe it is. There is surprisingly little solid evidence and few high quality studies on marijuana.
One of the problems with studying what has been until recently an illegal drug, is that it’s difficult to conduct a controlled, randomized clinical trial (RCT) where the intervention is to get the study group high (though I doubt this is due to a lack of volunteers).
As a result most cannabis studies are observational, where researchers identify historical correlations and try to control for confounding variables, which is notoriously difficult to do. The history of medicine is littered with such discarded correlations, which cause transient enthusiasm or alarm until ultimately proven false.
The lesson, which apparently has to be learned over and over again, is that correlation is not causation.
The weight of the evidence, however, does support the idea that heavy cannabis use during brain development (which lasts until age 25) increases the risk of psychosis and cognitive impairment, especially when other risk factors are present.
Here are three representative studies that give me pause.
The first is a study published in the journal Proceedings of the National Academy of Sciences in 2012. The authors analyzed data from the Dunedin cohort - an amazing New Zealand study that has been following more than 1,000 people for the past 50 years.
I like this study because it is longitudinal, using interviews and neuropsychological testing done on the same subjects from ages 13 through 38.
The authors found that cannabis was associated with a dose-dependent and statistically significant IQ decline, global neuropsychological decline, and functional decline in daily life. Furthermore, these effects were particularly pronounced in adolescents and not fully reversible with cessation of cannabis use.
The second is a study published in the Journal of Neuroscience in 2014. This study compared high resolution MRI scans of young adult recreational marijuana users with non-user controls.
I like this study because, while still observational, MRI findings are arguably more objective than interviews or IQ tests.
The authors found exposure-dependent changes between the two groups in both the structure and volume of the reward/aversion areas of the brain, supporting a biological basis of disease.
The third is a nationwide Danish study published in JAMA Psychiatry in 2021, looking for a connection between cannabis and schizophrenia.
I like this study because the inherent limitations of observational studies are offset here by the sheer number of subjects - Danes who were 16 years or older at some point between 1972 and 2016 - over seven million people.
The authors found a positive correlation between marijuana use and schizophrenia, with the proportion of cases of schizophrenia attributable to cannabis use disorder increasing by about 6-8% over the past two decades.
It is easy to get lost down the rabbit hole of studies and trials, especially when you can find experts on both sides of the debate. The truth is that we need better evidence to fully understand the risks weed poses to the developing brain.
But let’s not let the perfect be the enemy of the good. We know enough to say that the risks outweigh the benefits, which makes me uneasy about where we are headed as a society.
For my part, I tell my kids, and my young patients, to stay away from weed, as I did that day when Kobi pointed out the smell.
“I know the plan is to skip college and play goalie in the premier league,” I said, “but try to not kill off your brain cells anyway - you never know when you might need them.”
“Don’t worry.” he said, “Anyway, it’s still against the law to sell weed in NY.”
Is it, though?
We walked over to the truck in the picture, which has been parked on our block for over a month. I asked the guy what he had, and he handed me a laminated menu of cannabis products.
“I thought it was illegal to sell weed.” I said.
“Maybe it is,” he said, “but we’re not selling it.”
“OK, then how would one get it?” I asked.
“Well, first you buy this T-shirt,” he said, “then we give you the weed for free.”