Since its recreational legalization in 2016, marijuana has taken over Boston culture. 

From Hempfest — also known as the Boston Freedom Rally — on Boston Common to weed pop-up shops at festivals, the days of long and hard searches for marijuana are over, especially for young people. 

But research has shown how using marijuana at an early age can cause pronounced changes in developing brains. It affects the white matter of the brain, an essential network of nerve fibers that connect regions of the brain for fast communication. 

Dr. Staci Gruber, Director of Marijuana Investigations for a Neuroscientific Discovery at McLean Hospital, joined GBH’s All Things Considered host Arun Rath to share more about how cannabis consumption affects brain development. What follows is a lightly edited transcript of their conversation.

Arun Rath: First, to understand what we’re talking about — a little bit of basic brain anatomy. Tell us what white matter is and its relation to gray matter.

Dr. Staci Gruber: So, you know, I think of it this way: The brain is comprised of gray matter, white matter, and CSF, or cerebrospinal fluid.

Gray matter is really the hardworking sort of neurons, the part of the brain doing the job, so to speak. White matter connects brain regions for good, clean communication. I’ve heard it compared to things like the subway system, the electrical tape, the insulation — however you want to think of it.

You know, you can have all the hard-working gray matter in the world, but without the ability for one brain region to communicate with another, it won’t matter. White matter and the integrity of white matter are actually really rather critical.

Rath: What do we know so far about the effects of marijuana use on white matter?

Gruber: It’s one of those situations where I like to say, “age matters.” So many things make a difference here. In a study that we did in recreational consumers who either began using cannabis early — and in our studies that was prior to age 16 — and these were people who were using consistently, so regular, frequent cannabis use relative to those who were later [cannabis users]. That was sort of the comparison. So you have cannabis users in general, and then you have early and late users versus those who don’t use cannabis at all.

What we found with regard to white matter was that the early onset cannabis users had decidedly lower white matter coherence or organization relative to both the later onset folks and to the people who didn’t use cannabis at all. That’s really interesting. It really suggests that we know that the brain develops from the back to the front and from the bottom to the top.

During these critical periods of neurodevelopment, the brain is immature. It’s under construction or — if you’re a cannabis audience member — half-baked. So, we really want to be mindful of exposing the brain to things like exogenous or outside cannabinoids during a time where it may actually take a toll.

Rath: It sounds like a bad thing if the brain’s communication network is not coherent, as you say. How does that affect behavior and function?

Gruber: It’s a great question. And again, it’s not as if people can’t function and they’re not able to live their lives. But when we look at performance of different types of tasks, what we found was that from a cognitive perspective, the early onset folks — those who began using prior to age 16 — also had more difficulty with these tasks that require what we call “frontal executive function,” so the frontal cortex of the brain, just behind your eyebrows.

Having more difficulty doing those tasks underscores the importance of making sure that the brain is communicating — that each part is able to communicate effectively, if you will. The later-onset folks did not demonstrate the same patterns of challenge that the early-onset folks showed, so we look at it sort of in the aggregate.

Cognitive performance in conjunction with a structural brain measure suggests that early-onset or exposure [to cannabis] may be more deleterious than people might have imagined. And certainly, more deleterious than exposure at later ages, which we now know to be true, especially from other studies of older adults.

Rath: Is the significant difference really only for the early onset users, or are there implications for older users who may be using heavily?

Gruber: Again, when we look at — and a lot of studies do this — cannabis users versus non-users, and if you put everybody together, you might see differences. The important part of the work that we did years ago that really looked at early versus late users within the cannabis using group — we split them into those with early and later onset — it was only the early-onset folks who really demonstrated this difference with regard to lower white matter fiber tract coherence.

It does appear that individuals who begin using cannabis later in life, I would say well beyond the period of neurodevelopmental vulnerability — 40s, 45, when people are now worried about losing what they had, not the other way around — it appears actually to have an opposite effect, at least in some folks. They appear to have greater white matter coherence or organization.

Rath: We do have legal cannabis, but the legal age is 21. In terms of the health policy approach, how can we prevent younger users from getting hold of powerful marijuana to start off?

Gruber: Yeah, it’s a great question. You know, long ago and far away in our group, we started the mantra, instead of saying, “Just say no,” which did not work — these messages just tell people to never use it, absolutely not. We now say, “Just not yet.” Give your brain a chance to become fully developed before you expose yourself to something that may compromise things later for you. It’s a bit of an easier message for people to take.

With regard to the legal status, I will tell you that many of these studies in our group began well before we had legal cannabis for medical or recreational concerns or purposes. We didn’t have any trouble recruiting people, I will tell you that. No shortage of cannabis users at all ages in our state and really across the country.

It’s really a message of allowing people to understand that they’re giving themselves the best opportunity to have full brain development before they begin using products that may have a negative impact on their brain health.

Cannabis today is also not the same as cannabis of years ago. The higher amounts of Delta-9 THC, or tetrahydrocannabinol, the primary intoxicating compound in the plant, are what are so concerning and potentially damaging to the brain. It’s not many of the other constituents; Delta-9 is what we’re most concerned about most often. If we can just allow people to understand that, that’s really what I would call a big win.

Rath: We’ve seen reports that doctors say the number one reason young people are seeking addiction help is cannabis. We thought for a long time that cannabis was not addictive in the traditional sense. Is there a better understanding now of dependency?

Gruber: Yeah, you know, we have different terms these days, right? Our lexicon has changed, but your point is a very good one. We’re mindful that Delta-9 THC — so, cannabis itself, the plant — is a multi-compound plant. There are over 500 compounds in the plant, and 120 or so of these are phytocannabinoids, things from the plant that interact with our own endocannabinoid system. Delta-9 THC is the primary intoxicating compound. CBD is a primary, but not the only, non-intoxicating compound.

As it turns out, exposure to Delta-9 is important because THC is biologically reinforcing. That is, it activates reward pathways in the brain, and because of that, there are concerns about individuals becoming dependent or having a cannabis use disorder, where you have a pattern of misuse.

That is an important consideration these days for people, especially if they’re beginning to use [cannabis] in higher amounts and with greater frequency at earlier ages.