Does long-term marijuana use have a negative impact on IQ and function? New research from New Zealand seeks to answer this question.
Marijuana/Cannabis Use in the United States
Attitudes toward cannabis/marijuana use have changed significantly over the past twenty years in the United States. Regular use declined from the early 1980s to the early 1990s, increased during the 1990s, decreased again between 2001 and 2006, and has increased steadily since 2006.
These trends mirror both cultural changes and changes in the legal status of marijuana. The 1960s and 1970s saw an explosion of marijuana use across the country. Between 1980 and 2005, trends were unstable. Then, with the movement towards legalization – including medical and recreational marijuana – cannabis began to increase across all demographics.
Here’s a summary of the trend towards legalization and decriminalization of cannabis in the U.S. justice system.
Marijuana: Legal Status
- In the 1970s, 14 states decriminalized marijuana.
- In the 1990s, 21 states legalized medical marijuana.
- Between 2012 and 2022, a total of 19 states legalized recreational marijuana.
In total, recreational marijuana is now legal in 19 states and Washington, D.C. Medical marijuana is now legal in 38 states and Washington, D.C. This trend toward legalization mirrors public opinion. A poll of over 5,000 adults conducted in 2022 by the Pew Charitable Trusts shows the following.
Marijuana: Opinions on Legalization
- 10% of respondents said marijuana should be illegal in all cases
- 30% respondents said marijuana should be legal for medical use only
- 59% of respondents said marijuana should be legal for recreational and medical use
- 1% of respondents refused to offer their viewpoint
Public opinion on legalization also mirrors public perception of the harms of marijuana use. The National Survey on Drug Use and Health (NSDUH) reports the following.
Marijuana: Perceived Harm/Risk of Regular Use
- 2002: 51.3% of people 18+ perceived regular cannabis use as a great health risk
- 2012: 40.3% of people 18+ perceived regular cannabis use as a great health risk
- 2021: 25.7% of people 18+ perceived regular cannabis use as a great health risk
These trends beg a question. With the increased rates of use across all demographic groups and increased availability of marijuana for recreational use, what are the long-term consequences of long-term regular marijuana use?
Impact of Long-Term Cannabis Use on Cognition
A long-range study recently published in New Zealand includes important data for us to consider. The study “Long-Term Cannabis Use and Cognitive Reserves and Hippocampal Volume in Midlife” is significant for several reasons. First, it’s one of the only long-range studies on the impact of marijuana use on IQ and cognition with a stable control group against which researchers can compare results. Second, it assesses the impact of long-term cannabis use at midlife – around age 45 – which is important because this demographic group uses marijuana at relatively high rates. Third, it compares regular marijuana users with regular tobacco users and regular alcohol users in order to understand any similarities or differences between those groups.
Before we look at that study, let’s take a look at the current rates of marijuana use in the U.S.
Marijuana Use: Facts and Figures
Here’s the latest data on regular marijuana use in the U.S., as published in the 2021 National Survey on Drug Use and Health (2021 NSDUH):
Marijuana Use in the U.S.: 2021
- Total: 52.5 million
- 12-17: 2.7 million
- 18-25: 11.8 million
- 26+: 37.9 million
- Marijuana use disorder: 16.3 million
Here are the basic facts on the impact of regular marijuana use established before the publication of the study we introduce above. This information appears in a paper called Adverse Health Effects of Marijuana Use, by Dr. Nora Volkow, Director of the National Institute on Drug Abuse (NIDA).
Short-Term Marijuana Use: Cognitive Impact
- Impaired short-term memory
- Impaired motor coordination
- Altered judgment/decision making
- Can trigger psychosis at high doses
Consequences Of Long-Term or Heavy Marijuana Use
- Cannabis use disorder, or cannabis addiction:
- Occurs in 9% of people who use cannabis
- Risk increases with increased use
- Altered brain development, including:
- Negative impact on learning/memory
- Negative impact on executive function
- Problems with impulse control
- Problems with decision-making
- Negative impact on educational achievement
- Negative impact on cognition, memory, and IQ
That data – and that last bullet point in particular – offer ample justification for our interest in this latest study out of New Zealand. While the data presented by Dr. Volkow shows the consequences of long-term marijuana use, the study from New Zealand offers something the Volkow study doesn’t: a 40-year analysis of the impact of marijuana use on IQ and cognition, within a single group of participants.
Cannabis Use, IQ, and Memory: The Results
Here’s how the research team describes their study:
“This study tested the hypotheses that long-term cannabis use is associated with cognitive deficits and smaller hippocampal volume in midlife, which is important because midlife cognitive deficits and smaller hippocampal volume are risk factors for dementia.”
That’s another reason this study is important. Not only does regular marijuana use lead to significant long-term health problems, it can also increase the risk of dementia. This is of particular concern, because the population of the U.S. is aging, with the number of people over age 65 on pace to reach 95 million by 2060, almost double the population of people over 65 now.
With that in mind, let’s take a look at the study participants and parameters:
- Participants included 1,037 individuals born in Dunedin, New Zealand, in 1972–1973
- The research team followed the group until age 45 and collected data on cannabis use and dependence at ages 18, 21, 26, 32, 38, and 45.
- Researchers assessed intelligence quotient (IQ) at age 7, 9, 11, and 45.
- Researchers assessed neuropsychological functions and hippocampal volume at age
- 94% of the group completed the entire study.
The research team compared long-term cannabis users against five different groups.
1. Cannabis Non-Users
Participants in this group never used cannabis, never received a diagnosis for substance use disorder (SUD), and never used tobacco daily.
2. Midlife Recreational Cannabis Users
Participants in this group use cannabis between 6 and 51 days a year at ages 32, 38, and 45, but no history of weekly cannabis use or cannabis dependence.
3. Long-Term Tobacco Users
Participants in this group smoked tobacco daily at age 45 and in one or more previous time points, but reported no history of weekly cannabis use or cannabis dependence.
4. Long-Term Alcohol Users
Participants in this group reported weekly drinking at age 45, diagnosis of alcohol dependence at two or more time points, and no history of weekly cannabis use of cannabis dependence.
5. Cannabis Quitters
Participants in this group reported no cannabis use at age 45, but had a previous diagnosis of cannabis dependence, or reported regular – i.e., more than 4 days per week – cannabis use at earlier time points.
The non-user group acted as a de facto control group for all groups. Researchers created the midlife recreational group to determine whether non-problem use in midlife impacted hippocampal volume or cognition. Researchers created the tobacco user and alcohol user groups to identify and disentangle the consequence of tobacco and alcohol use from the consequences of cannabis use. Finally, researchers created the cannabis quitters group to determine whether changes persisted after cessation of use.
Results: Mean Intelligence Quotient (IQ)
- Long-term cannabis users:
- Child IQ: 99.3
- Adult IQ: 93.8
- Change: Decreased by 5.5 points
- Cannabis non-users:
- Child IQ: 101.4
- Adult IQ: 102.1
- Change: Increased by 0.7 points
- Midlife recreational cannabis users:
- Child IQ: 105.1
- Adult IQ: 101.6
- Change: Decreased by 3.5 points
- Long-term tobacco users:
- Child IQ: 93.0
- Adult IQ: 91.5
- Change: Decreased by 1.5 points
- Long-term alcohol users:
- Child IQ: 99.3
- Adult IQ: 98.8
- Change: Decreased by 0.5 points
- Cannabis quitters:
- Child IQ: 97.6
- Adult IQ: 94.3
- Change: Decreased by 3.3 points
That data confirms the negative impact of long-term marijuana use on intelligence quotient. Compared to all other groups, regular cannabis users lost an average of 5.5 IQ points between childhood and adulthood, compared to the loss of less than one point for cannabis non-users. It’s also important to note the loss of 3.5 points by midlife cannabis users and 3.3 points by cannabis quitters.
Next, the data on cognition.
Results: Cognitive Tasks
(Higher scores indicate worse memory and attention)
- Long-term cannabis users:
- Memory: 0.53
- Attention: 0.56
- Difference: Significantly higher than all other groups
- Cannabis non-users:
- Memory: -0.19
- Attention: -0.10
- Difference: Significantly lower than long-term cannabis users, midlife recreational cannabis users, long-term tobacco users, and cannabis quitters
- Midlife recreational cannabis users:
- Memory: 0.03
- Attention: 0.00
- Difference: Lower than long-term cannabis users (both metrics), lower than long term tobacco users (both metrics), higher than cannabis non-users (both metrics), similar to long-term alcohol users for memory, higher than long-term alcohol users for attention, lower than cannabis quitters
- Long-term tobacco users:
- Memory: 0.19
- Attention: 0.11
- Difference: Lower than long-term cannabis users, higher than long-term alcohol users, similar to cannabis quitters for memory, lower than cannabis quitters for attention
- Long-term alcohol users:
- Memory: 0.05
- Attention: -0.16
- Difference: Higher than cannabis non-users, lower than long-term cannabis users, lower than long-term tobacco users, similar to midlife recreational cannabis users for memory, lower than midlife recreational cannabis users for attention, lower than long-term tobacco users for attention
- Cannabis quitters:
- Memory: 0.18
- Attention: 0.23
- Difference: Lower than long-term cannabis users, higher than midlife recreational cannabis users, similar to long-term tobacco users for memory, higher than long-term tobacco users for attention, higher than cannabis non-users
This data confirms previous reports on the negative impact of long-term cannabis use on two key cognition metrics, memory and attention. Long-term cannabis users performed significantly worse than all other groups in the study on these metrics. Of note here is that long-term alcohol users and midlife recreational cannabis users showed similar results, while midlife recreational cannabis users showed significantly better scores that cannabis quitters.
Finally, let’s look at the data on changes in the structure of the brain associated with long-term cannabis use, as measured by the volume of the hippocampus
Results: Hippocampal Volume
Note: the hippocampus is an integral part of the limbic system in the brain, which plays a pivotal role in regulating emotions and emotional response to external stimuli.
- Long-term cannabis users:
- Difference: Significantly lower volume than cannabis non-users and midlife recreational cannabis users
- Cannabis non-users:
- Difference: Significantly larger volume than long-term cannabis users
- Midlife recreational cannabis users:
- Difference: Significantly larger volume than long-term cannabis users
- Long-term tobacco users:
- Difference: No statistically significant difference, compared to long-term cannabis users
- Long-term alcohol users:
- Difference: No statistically significant difference, compared to long-term cannabis users
- Cannabis quitters:
- Difference: Statistically larger volume in one hippocampal area – the HATA – compared to long-term cannabis users
Those are all the relevant results of this study. Taken together, they confirm associations indicated in previous research. This study, however, carries added weight because of the study design: researchers had several group for comparison in one cohorts, and maintained 94% over almost 50 years. That, in an of itself, is a novel accomplishment – and the data the research team collected offers us crucial insight into the impact of long-term cannabis use on IQ, memory, and attention.
How This Research Helps
We can all recognize that in 2023, then general attitude toward cannabis/marijuana use and consumption is one of tolerance and acceptance. Some groups – notably people over age 75 – still disapprove of marijuana legalization on all levels. However, the trend for most groups over the past twenty years tends toward a reduced perceived risk of harm and an increased acceptance of both medical and recreational use of marijuana.
This research helps give current and potential marijuana users facts upon which to base their behavior. While the legalization and tolerance trend may also include a trend toward thinking marijuana is completely harmless, the data we present above suggests otherwise. Long-term use can lead to significant deficits in IQ, memory, and attention. Older adults who have engaged in long-term use should understand the impact cannabis use may have had on their brain structure and function. Younger, new users should understand the potential negative impact of long-term cannabis use on brain structure and function.
As cannabis use increases nationwide, and the recreational legalization trend continues, it’s critical that everyone has the latest evidence on the negative impact of long-term cannabis use on IQ and cognitive function, in order to make informed, evidence-based decisions on cannabis use, whether medical or recreational.