8 Things To Know About Cannabis and the Brain
How past marijuana use impacts memory at midlife
Hello, everyone. I am writing to you from Denver as I wait for my son to come out of knee surgery. I know what you are thinking: didn’t YOU just have knee surgery? Yes, it’s true. I am six weeks post-op from a total knee replacement. I am doing really well and so is my dog Orzo, who pulled through a health crisis recently. Thank you for all the sweet messages asking after Orzo and me. The BHK community is simply the best.
Today I am sharing some new information about the impact of cannabis use on the brain. Perhaps the most common question I get asked about cannabis is whether past use is bad for the brain. In other words, does smoking a lot of marijuana as a young adult increase the risk of dementia later?
First, let’s just say that this is a topic where there are still more questions than answers. But now, thanks to the largest study ever done in young adults, we are gaining more clarity.
I’ll summarize this study and a few others for you below and share some surprising facts about cannabis use and the brain. And I’ll fill you in on what I’ve been up to—including our next upcoming BHK cooking class.
This first newsletter of the month is always free. Please share!
Cannabis Use, Defined
Let’s get this out of the way first: there’s no need to panic if you smoked pot every once in awhile decades ago. As far as we know, minimal use has not been shown to impact brain health at midlife. Studies refer to marijuana use over years, also called cannabis exposure. Here’s how the studies define it:
Heavy use: using marijuana more than 1,000 times over one’s lifetime. This sounds like a lot, but could be someone who smoked pot three times a week over five years or once or twice a week over 10 years.
Moderate use: using marijuana between 11 and 999 times over one’s lifetime. This could be someone who used marijuana once a week or once every other week over 10 years.
Nonusers: using marijuana less than 10 times over one’s lifetime.
As you can see, occasional use over many years counts as heavy use. What about consuming other cannabis products (such as a THC or CBD edible) every night for 10 years? This hasn’t been studied. (See #6, below.) We also don’t know the different brain health impacts of smoking, vaping, or consuming cannabis in a pill or gummy form.
8 Things To Know About Cannabis and the Brain
Heavy marijuana use impairs memory skills 5 years after quitting. Up until now, it wasn’t clear if using cannabis in the past had a lasting impact on brain health. Now we know that it does, thanks to this study in JAMA Network of more than 1,000 cannabis-using young adults between ages 22 and 36. Researchers followed their brain health with MRI brain imaging and cognitive testing for five years after quitting. They wanted to see how cannabis use impacts cognitive function later in life, and what that looks like on brain scans.
Here’s what they found:
Heavy users had more difficulty with tasks that required working memory. You tap into this cognitive skill when doing math in your head, remembering a phone number long enough to dial it, and keeping track of a conversation while planning a response.
Heavy users had significantly lower amounts of brain activity in certain brain regions (the prefrontal cortex and the anterior insula) during working memory tasks.
Heavy users did not have impairment in other functions like motor skills, reward processing, emotion, and language.
Moderate or less users did not have lasting impairment in cognitive function.
2. Heavy marijuana use as a young adult can impair verbal memory at midlife, a risk factor for dementia. Does the memory impairment that comes with heavy marijuana use show up 25 years later? According to this study, it does. The CARDIA (Coronary Artery Risk Development in Young Adults) study was the first large study to look at marijuana use and long-term brain health—3,500 participants over 25 years. Researchers tracked marijuana use in participants aged 18 to 30 years, and followed their cognitive function 25 years into the future. The researchers wanted to learn if past exposure to marijuana had a measurable impact on things like verbal memory and processing speed long after quitting. You tap into verbal memory when you are trying to recall words from a list, such as a shopping list.
Here’s what they found:
The 9% of participants still consuming marijuana into midlife performed the most poorly on all the cognitive tests they looked at: verbal memory, processing speed, and executive function.
Past user’s performance on cognitive tests depended on how much marijuana was consumed over the years before quitting. The greater the exposure to marijuana, the worse their verbal memory.
There’s a dose-dependent association between how much marijuana was consumed as a young adult and worsening verbal memory at midlife. Each additional 5 years of marijuana exposure knocked an additional 1 to 2 points off the ability to recall 15 words from a list.
Past marijuana use did not have an impact on executive function or processing speed at midlife.
3. Cannabis exposure shrinks the brain’s memory center over time. Perhaps the most compelling data published on this topic looked at what happens to the hippocampus—the brain’s memory center—over time when chronically exposed to cannabis. This study from New Zealand followed over 1,000 cannabis users from birth to age 45.
Here’s what they found:
Long-term cannabis users showed smaller hippocampal volume, even after correcting for other factors known to shrink the brain, such as tobacco and alcohol use. Not surprisingly, maintaining a larger hippocampus with age was also associated with doing better on memory tests.
4. Long term cannabis users lose an average of 5.5 IQ points from childhood to midlife. In the same study that measured hippocampi volume using MRI, researchers tracked IQ in cannabis users from childhood to age 45.
Here’s what they found:
Long-term cannabis users lost an average of 5.5 IQ points from childhood to midlife. This translated to poorer processing speed and memory and attention problems throughout life.
When compared to long term tobacco or alcohol users, long term cannabis users fared significantly worse—with a larger IQ decline, poorer learning and memory, and poorer processing speed.
4. Men may have more verbal memory impairment than women from using cannabis. According to this study looking at male and female sex differences on brain health, males may be more vulnerable to lasting cognitive problems than women. It’s widely known that women and men possess different verbal skills. This is one reason why women often slip through the cracks of dementia screening tests—superior verbal skills help them perform better, thus getting diagnosed later.
5. Occasional or light marijuana use has not been shown to have lasting impact on the brain. If you are worried about occasional past exposure to cannabis earlier in life, rest assured. None of the studies I reviewed showed that light or occasional marijuana led to lasting impairment. This was found only in heavy users.
6. The long term brain health risks of consuming cannabidiol (CBD) and tetrahydrocannabinol (THC) are not known. Do we know the long term brain health impact of consuming cannabidiol (CBD) and tetrahydrocannabinol (THC) products for medicinal or recreational use? No, we don’t, because these products have not been studied in humans long term. We don’t know if CBD gummies, for example, are innocuous or possibly beneficial for brain health. Or, if long term consumption could push someone closer to a diagnosis of dementia.
That’s crazy given the wide use of CBD and THC products. In a recent survey of women published in the journal Menopause, 86% of respondents were currently using a cannabinoid product to treat peri- or postmenopausal symptoms, such as vasomotor symptoms and sleep disturbances.
7. Cannabinoids have potential for treating agitation in people living with Alzheimer’s. In this study out of Johns Hopkins University School of Medicine, researchers wanted to see if a cannabinoid derivative could help Alzheimer’s patients suffering from agitation. Participants were given either a placebo or a pill form of the drug dronabinol, an FDA-approved synthetic version of THC, and followed over 8 years. Dronabinol is FDA-approved to treat loss of appetite in patients with HIV/AIDS, and is currently prescribed to treat nausea and vomiting in those undergoing cancer chemotherapy.
Here’s what they found:
When compared to a placebo, dronabinol reduced agitation in patients with Alzheimer’s by an average of 30%.
Dronabinol produced similar calming effects as current treatments for agitation, such as antipsychotics, but without adverse results such as delirium or seizures.
8. Is cannabis use a risk factor for dementia? We can’t say this with certainty. Even though there are a handful of long-term, well-designed studies showing that heavy past cannabis use actually changes the structure of the brain and leads to cognitive impairment decades later, we still don’t know if that translates to greater dementia risk. Researchers would have to design even longer studies that span from young adulthood into the high-risk decades for Alzheimer’s and dementia. As is true with much research, association does not prove cause-and-effect.
If you’ve been reading this newsletter or are well-versed in brain health topics, I am sure you can connect the dots. A substance that impairs memory, is strongly associated with a drop in IQ, and shrinks the memory center of the brain is highly likely to accelerate age-related cognitive decline. But more studies will need to be done before cannabis exposure is added to Lancet’s 14 modifiable risk factors for dementia, along with tobacco use, heavy alcohol use, high blood pressure, diabetes, elevated LDL cholesterol, and many others.
Thoughts?
Besides endless PT sessions for my knee, here’s what else I’ve been up to:
Teaching undergraduates and graduate students. The Women’s Brain Health course at Harvard is in full swing! It’s been an amazing experience to collaborate with lead faculty Drs. Stephanie Peabody and Shelly Carson on this first-of-its-kind course. Next up, I’ll be diving into creating the coursework for the first-ever Brain Health Nutrition course at the University of Cincinnati.
Over on Substack Notes, I’ve been giving a daily Brain Health Pop Quiz. Check it out and see how you do!
Planning spring Brain Health Retreats. I am knee-deep in travel plans as I head to Puglia, Italy for Food, Fun, and Brain Health, a retreat in collaboration with
and ViaRosa Italy. Besides doing yoga each morning amongst the olive trees, we’ll be exploring the heel of the boot of Italy to be fully immersed in the Mediterranean lifestyle. Then, I’m off to Sardinia for a trip to Italy’s Blue Zone: Sardinia’s Secrets for a Long and Healthy Life, in collaboration with Academic Travel Abroad. These retreats are sold out, but don’t worry--I have more coming up this fall. (See below.)Center for BrainHealth. It was an immense honor to speak last week at the Center for Brain Health’s first female-focused brain health conference to speak about what to eat before, during, and after perimenopause. I joined some of my favorite physicians and scientists like Dr. Jessica Shepherd, author of Generation M and Dr. Matt Walker, sleep expert and author of Why We Sleep. I’ll be posting the recording soon. Meanwhile, be sure to get on their list to receive links to their free speaker series Brain Health Presents.
Getting back into the kitchen to test recipes for the next FM cooking class. Recovering from knee surgery has definitely meant less time in the kitchen! Now that I’m back on my feet, I’ve been enjoying brainstorming new recipes for the next BHK online cooking class on March 23. Everyone is welcome to join these quarterly live-on-zoom cooking classes where we also discuss a brain health topic. To sign up, upgrade your subscription to Founding Member by March 16, 2025 and I’ll send you the zoom link and the recipes. Each class costs just $35 if you are already a paying subscriber.
That’s all for today. I hope your weekend is off to a great start. As always, thank you for reading, sharing, and taking the best care of your brain. I’ll be back next week with new information about very early dementia screening and, if all goes well in the kitchen, a new recipe. In the meantime, come say hello over on Notes.
Love,
Annie
Brain Health, Food, and Fun in Puglia, Italy in April, 2025: SOLD OUT.
Sardinia, Italy in May, 2025: SOLD OUT. Join the waitlist for this fall’s trip by calling 877-298-9677.
BHK Retreat at Rancho La Puerta, Mexico in June 2025: Announcing dates soon.
Costa Rica in February 5 to 10, 2026: We are already half full. Learn more and reserve your spot here.
Hi Annie - thanks so much for this article! I’m trying hard to get off my sleep gummy (which has equal parts CBD/CBN/THC) but haven’t been successful, yet. And I know not sleeping is very problematic in terms of brain health. So it’s hard to know which is worse, taking the gummy or not sleeping! 🤷♀️
You did it again Annie! Every time I’m interested in a topic, you come out with a post about it!! What I was wondering is with the know detrimental effects of alcohol, and all the new thc infused cocktails, would that be a better option on occasion?