What do Carl Sagan, Bill Gates, Steve Jobs, and a trio of Nobel Prize-winning scientists have in common? They are all marijuana-using intellectuals who defy both the enduring stereotype of the ‘stupid stoner’ and the cautionary trope that marijuana will ‘rot your brain.’
The effects of cannabis on human brain development, cognition, and intelligence remains an often debated and heavily researched topic. And given the increasing availability and popularity of both medical and recreational cannabis, it is a subject that deserves the attention of anyone contemplating medical marijuana therapy.
Marijuana & Brain Development
The short-term cognitive effects of marijuana are well known, and commonly include impairment to short-term memory, changes in time and distance perceptions, and decreased reaction times. The extent of these and other reactions to marijuana use can vary significantly from one individual to another, but typically result in a window of transitory impairment of between 3 and 10 hours.
Less well understood are the long-term effects of marijuana use on higher level cognitive functions such as learning, memory, problem solving, decision making, and attention. Studies attempting to answer these questions typically track subjects for a period of years and even decades. A review of this literature reveals that while early research on the subject hints at the possibility of neurological damage from marijuana use, these warnings are seldom found in subsequent studies that model for developmental factors other than marijuana use.
A 2012 study that tracked more than 1,000 habitual marijuana users through age 38 reported an observed long-term trend towards neuropsychological decline in the sample population, while noting that these effects were associated with adolescent-onset cannabis use, and more pronounced among heavy users. The authors warned that their findings are “suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents.”
Conflicting findings were drawn from a more recent study of some 800 adolescent marijuana users tracked from age 14 to 19, which reported no measured impact on decision-making abilities among moderate users who began marijuana use after the age of 15.
The authors stated:
We find no evidence to support the presumption that cannabis consumption leads to a decline in neurocognitive ability.
A 2018 study that assessed both cognitive performance and brain development in teen-onset marijuana users also failed to find a link between cognition and marijuana use. In their concluding remarks the authors stated:
Our results do not support the idea that early cannabis initiation alone predicts cognitive dysfunction by age 15.
The researchers went on to explain that:
Purported neurocognitive effects of early cannabis onset may not be due to cannabis initiation alone, but also driven by limitations or late development of neurocognitive systems predictive of substance use.
One of the challenges of studies on the effects of long-term use of marijuana is the need to identify and eliminate, or compensate for, the numerous variables apart from marijuana use that might play a role in cognitive development and maintenance. Numerous studies have attempted to determine if teenage marijuana smoking leads to behavioral problems, lack of motivation, and decreased learning, or if these same counter-productive behaviors increase the likelihood of teenage marijuana use. In simple terms, it’s a case of: “Does marijuana smoking make for bad kids, or do bad kids smoke marijuana?”
Twin studies represent the gold standard for many types of behavioral and physiological studies. The use of identical siblings as subjects can eliminate a range of environmental, genetic and social variables, allowing for the isolation of a single or limited number of control factors — in this case, the use of or abstinence from marijuana.
A review of twin studies focused on the possible effects of marijuana on cognitive function noted some alterations to cognitive abilities based on continued use of marijuana, but failed to draw an association between cannabis use during adolescence and a subsequent change in young adulthood cognitive abilities. The study concluded:
We found little support for a potential causal effect of cannabis use on cognition, consistent with previous twin studies. Results suggest that cannabis use may not cause decline in cognitive ability among a normative sample of cannabis users.
A Vietnamese study compared the long-term cognitive performance of twin pairs where only one sibling had a history of marijuana use, followed by a cessation of use for periods ranging from one to 20 years. Based on the results of a comprehensive neuropsychological test battery to assess general intelligence, the study came to the conclusion that there was an “absence of marked long-term residual effects of marijuana use on cognitive abilities.”
More Cannabis & Cognition Studies
A long-term study of adult twin pairs in England and Wales reported:
- Compared with adolescents who did not use cannabis, adolescents who used cannabis had lower IQ in childhood prior to cannabis initiation and lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from ages 12-18.
- Adolescents who used cannabis had poorer executive functions at age 18 than adolescents who did not use cannabis, but these associations were generally not apparent within twin pairs.
- Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence.
- Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests.
There is also evidence that when diminished cognitive performance is observed, it may be transitory in nature, based on current and ongoing use of marijuana. A quantitative synthesis of literature focused on cannabis use and cognitive functioning in adolescents and young adults concluded that:
- Associations between cannabis use and cognitive functioning in cross-sectional studies of adolescents and young adults are small and may be of questionable clinical importance for most individuals.
- Abstinence of longer than 72 hours diminishes cognitive deficits associated with cannabis use.
- Results indicate that previous studies of cannabis in youth may have overstated the magnitude and persistence of cognitive deficits associated with use.
- Reported deficits may reflect residual effects from acute use or withdrawal.
More evidence that observed cognitive declines associated with marijuana may be a temporary use-based phenomenon rather than a permanent neurological change comes from an Australian study. The researchers followed mid-20s marijuana users for an eight-year period, employing a periodic battery of tests to assess changes in cognitive performance over time between marijuana users, non-users, and users who reduced their frequency of consumption. The most interesting conclusions to emerge from this research were that:
- Former heavy users improved cognitive performance as compared to continued heavy users.
- Cessation of cannabis use appears to be associated with an improvement in capacity for recall of information that has just been learned.
- No other measures of cognitive performance were related to cannabis after controlling for confounds.
An investigation into the long-term effects of frequent cannabis use on working memory and attention used a combination of verbal assessments and magnetic resonance imaging (MRI) to assess mental acuity and brain activity. A control group of non-users was assessed in comparison to frequent marijuana users who abstained from use for a period of one week.
The authors concluded:
No evidence was found for long-term deficits in working memory and selective attention in frequent cannabis users after one week of abstinence.
Does Marijuana Make You Stupid?
Forrest Gump’s assertion that “stupid is as stupid does” is actually not a bad way of explaining IQ scores, as these measures of human intelligence are based on testing regiments that assess mental acuity based on a person’s response to a specific battery of standardized tests. IQ testing has proven to be a valuable tool in the ongoing debate over marijuana’s long-term effects on intelligence.
At first reading, there might be some cause for concern from statements made in a 2017 study published in Drug and Alcohol Dependence, which drew conclusions from data recorded in the National Longitudinal Study of Adolescent to Adult Health. This national survey of more than 20,000 adolescents is based on a combination of academic performance assessments and individual interviews. After crunching the numbers, the authors reported what they identified as a “small but detectable difference” of 1 to 2 points in intelligence between adults who had used marijuana as adolescents as compared to those who did not.
It should be noted that this association was made on a purely binary basis for use/non-use of marijuana, with no attempt to identify a range of confounding variables that might also influence outcomes. In addition, the authors ultimately concluded:
Our findings did not reveal a significant association between cumulative marijuana use and changes in intelligence scores.
A more definitive answer comes from a study of more than 2,200 teenagers who were measured for IQ at age 15 and academic performance at age 16. Results were adjusted for confounding variables such as cigarette and alcohol use, childhood mental health symptoms, and behavioral problems.
The authors reported:
After full adjustment, those who had used cannabis ≥50 times did not differ from never-users on either IQ or educational performance. These findings suggest that adolescent cannabis use is not associated with IQ or educational performance once adjustment is made for potential confounds, in particular adolescent cigarette use.
Modest cannabis use in teenagers may have less cognitive impact than epidemiological surveys of older cohorts have previously suggested.
Outcomes from a Canadian study of marijuana’s impact on intelligence compared IQ scores of both marijuana-using and abstaining subjects. Tests were administered at ages 9 to 12 (prior to marijuana use) and at ages 17 to 20. The results indicated that there was a measurable decline in IQ scores among current heavy users, as compared to increased IQ scores for light current users, former users, and non-users. The authors also noted that negative IQ effects were not observed among subjects who had previously been heavy users but were no longer using marijuana.
This led to the conclusion that:
Marijuana does not have a long-term negative impact on global intelligence.
A study conducted at the University of Southern California tracked a sample of more than 600 twins and triplets from age 10 to age 20, using a combination of biennial assessment criteria and IQ testing for all subjects at enrollment in the study and at age 20. Variables used in the study model included use of marijuana, alcohol, and other drugs. Based on this modeling, the conclusions drawn provide added strength to the argument that marijuana use may not be the reason for poor performance and lower IQ outcomes.
The authors emphasized this point by stating:
We find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline.
Researchers went on to explain:
The lack of a dose-response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana.
Will Marijuana Rot Your Brain?
Numerous studies have addressed the concern over marijuana’s potential influence on brain development and potential alternations to brain morphology. Among the areas of concern are volumetric changes in gray and white matter, as well as alterations to cortical thickness. Gray matter regions of the brain are responsible for processing information, while white matter provides communication links between various cerebral regions. Cortical thickness is generally considered to be a key measure of intelligence.
Two recent comprehensive reviews addressed the question of marijuana’s potential effect on gray matter development in adolescents. Researchers compiled information from 16 studies and 24 studies, respectively. Both sets of studies indicated a lack of evidence to support the hypothesis that adolescent-onset use of cannabis resulted in significant reductions in gray matter volume among cannabis users, while also noting that any variations in brain volume noted by individual studies were likely attributable to factors other than marijuana use.
Information from a study released in the January 2022 edition of Addiction Biology compared white matter volumes of 39 daily cannabis users to 28 non-users closely matched for age, sex, alcohol use, and other variables, while noting that previous studies on the subject had likely drawn inconclusive results from failing to account for all variables. The authors mentioned some minor abnormalities in tissue composition among early-onset users that might warrant further study, but came to the overall conclusion that:
White matter microstructure did not differ between cannabis users and controls and did not covary with recent cannabis use, dependence severity, or duration of use. These findings suggest that long-term near-daily cannabis use does not necessarily affect white matter microstructure.
Cortical composition and thickness was the subject of a multi-site MRI study challenging the assumption that marijuana use contributes to structural alterations across the cortical surface. Researchers mapped and compared the cortical thickness, surface area, and gyrification index (layering) of 141 cannabis users and 120 non-users.
Based on the results of these comparisons, researchers stated:
- Our results revealed that cortical morphology was not associated with cannabis use, dependence, or onset age.
- We additionally found no association between cortical surface area and gyrification index in relation to cannabis use.
- Our lack of finding in a well powered study suggests that cortical surface morphology may be less associated with cannabis use than previously assumed.
A report described as the “largest exploratory analysis to date” on the subject of brain imaging and cannabis use was published in the April 2018 edition of Addiction. Report findings were based on two large-scale studies of cannabis-using and non-using young adult twins and middle-aged twins. All subjects were subjected to MRI mapping and volumetric segmentation modeling to calculate the size of seven distinct subcortical regions of the brain (thalamus; caudate nucleus; putamen; pallidum; hippocampus; amygdala; and nucleus accumbens). Based on the results, the report concluded that there was:
[N]o effect of cannabis use on the volume at any subcortical region of interest in young adults or middle-aged males.
Concerns over the effects of life-long marijuana use on the brain were addressed in a pilot study of older adults in the mid-60s age range. A combination of MRI assessment and cognitive battery testing was used to compare brain morphology and mental acuity of non-marijuana users to individuals who reported an average of 23 years of regular marijuana use.
The researchers found no significant differences between these two groups, and reported:
These results suggest that cannabis use likely does not have a widespread impact on overall cortical volume.
A similar study reported in the American Journal of Addictions used MRI scans to compare older non-cannabis users with peers of a similar age who reported an average of more than 20,000 cumulative uses of marijuana (the equivalent of smoking one joint each day for more than 50 years).
In a summary of results, the authors stated:
When compared to control subjects, smokers displayed no significant adjusted differences in volumes of gray matter, white matter, cerebrospinal fluid, or left and right hippocampus. Moreover, hippocampal volume in cannabis users was not associated with age of onset of use not total lifetime episodes of use. These findings are consistent with recent literature suggesting that cannabis use is not associated with structural changes within the brain as a whole or the hippocampus in particular.
While there is little evidence that marijuana use hinders brain development, significant evidence indicates that adolescent brain development is hindered by alcohol use. A study appearing in the journal Addiction used neurological imaging to compare groups of adults and adolescents segregated into marijuana-using and alcohol-using groups. The researchers looked at potential changes to brain gray or white matter based on use patterns, and came to the following conclusions:
- Adult alcohol users had significant reductions in gray matter volume, as did adolescents to a lesser extent.
- Adult alcohol users exhibited poorer white matter integrity, but adolescent alcohol users did not.
- No associations were observed between structural measures and past 30-day cannabis use in adults or adolescents.
A study of more than 400 twins aged 24 used MRI scans to compare the key areas of the brain for cortical thickness. According to the authors, the results indicated that:
Greater alcohol, but not cannabis, misuse was associated with reduced thickness of prefrontal and frontal medial cortices, as well as temporal lobe, intraparietal sulcus, insula, parietal operculum, precuneus, and parietal medial areas.