Cannabis is the most commonly used psychotropic drug in the world. Because so many young people use cannabis, it is important to understand its short- and long-term side effects. However, due to conflicting research findings, there remains widespread debate about the impact that moderate-to-heavy cannabis use has on the developing brain and cognitive functioning, writes Christopher Bergland for Psychology Today.
Bergland shares a handful of evidence-based studies, a few anecdotal stories, and some meta-analyses that exemplify why there's so much confusion and uncertainty about how cannabis use during teenage years impacts on cognitive performance.
After establishing the lack of scientific agreement on the neurocognitive effects of cannabis use during adolescence, he looks at a University of Colorado Boulder study on this topic published on 3 September in the journal Addiction.
First author Jarrod Ellingson, assistant professor of psychiatry at Colorado’s School of Medicine, and co-authors explain the impetus behind their latest sibling comparison study on the effect of moderate‐to‐heavy cannabis use on cognitive functioning in adolescents:
"Due to changes in the legality of recreational and medical cannabis and widespread access in many states, valid empirical data must be available to inform policy and public health decisions, including how cannabis use may affect the developing brain."
A hot-button issue
The decision by some United States jurisdictions to legalise or decriminalise marijuana is a hot-button issue. Even though the possession of cannabis is illegal under federal laws contained in the Controlled Substances Act of 1970, the legality of marijuana varies from state to state.
One psychological consequence of legalisation and decriminalisation is that the general public increasingly assumes that marijuana use is ‘harmless’, ‘safe’, and, in some cases, ‘good for you’. Many people view cannabis as having medicinal benefits and, to a large degree, it's lost its ‘reefer madness’ stigma as a dangerous illicit substance.
In my opinion, destigmatising marijuana use isn't necessarily a bad thing, but widespread acceptance also creates a ripple effect of complicated consequences.
For example, reports from the annual Monitoring the Future (MTF) survey of drug use and attitudes among middle and high school students in the United States shows that as teens' perceptions about the risks of marijuana have steadily decreased over the past decade, moderate-to-heavy cannabis use among adolescents has steadily increased in tandem.
In 2019, the nationwide MTF survey found that 11.8% of 8th graders in the US reported using marijuana in the past year and 6.6% had used it in the past month. Among 12th graders, 35.7% said they'd used marijuana during the year prior to the survey, 22.3% had used it in the past month, and 6.4% said they used marijuana daily or almost every day.
Cognitive deficits may have been overstated
A few years ago, one of the largest longitudinal twin studies (Jackson et al 2006) to investigate the impact of adolescent marijuana use on intelligence quotient (IQ) scores found "little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline."
Another recent systematic review and meta-analysis (Scott et al 2018) asked: "Is frequent or heavy cannabis use associated with cognitive dysfunction in adolescents and young adults?"
After reviewing 69 studies that involved 2,152 cannabis users with an average age of 20.6 years old, J Cobb Scott and co-authors concluded that "previous studies of cannabis [use] in youth may have overstated the magnitude and persistence of cognitive deficits associated with use."
On the flip side, numerous studies have identified a link between heavy marijuana use, poorer cognitive functioning, and memory impairments. For example, another review (Jacobus and Tapert 2014) of the effects of cannabis on the adolescent brain concluded that "teens who engage in heavy marijuana use often show disadvantages in neurocognitive performance, macrostructural and microstructural brain development, and alterations in brain functioning."
About five years ago, a Molecular Psychiatry study (Riba et al 2015) on cannabis use and memory function concluded that "cannabis users have an increased susceptibility to memory distortions even when abstinent and drug-free, suggesting a long-lasting compromise of memory and cognitive control mechanisms involved in reality monitoring."
As you can see, there aren't any universally agreed upon, concrete conclusions about how marijuana use affects memory and cognition. Unfortunately, all of this seemingly contradictory empirical data makes well-informed policy-making decisions especially challenging for public health advocates.
Cannabis and verbal memory
Because there's so much indefiniteness surrounding the impact that cannabis use during adolescence has on cognitive functioning, Ellingson's team at the University of Colorado School of Medicine designed a sibling comparison study that looked at the effect of moderate‐to‐heavy cannabis use on cognitive functioning in adolescents over a seven-year period (from age 17-24).
"We wanted to expand our understanding of whether cannabis use is related to lower cognitive functioning," Ellingson said in a news release. "There's a large body of evidence that cannabis use is linked to cognitive functioning, but we know that cannabis use is not isolated from other important risk factors.
“That was the primary motivation behind this study, in which we compared siblings to account for many of these risk factors."
Some findings
This study included 1,192 adolescent participants from 596 different families. Marijuana use was assessed via clinical interviews; cognitive abilities were measured using a neuropsychological battery of tests.
The researchers collected two waves of data. The first wave was collected from 2001 to 2006, when participants had an average age of 17; the second wave of data was collected from 2008 to 2013, when the study participants' average age was 24.
Ellingson et al found that a "greater frequency and earlier onset of regular cannabis use were associated with poorer cognitive performance, specifically on tests of verbal memory." Based on these findings, the authors conclude: "Moderate adolescent cannabis use may have adverse effects on cognitive functioning, specifically verbal memory, that cannot be explained by familial factors."
"More work needs to be done to determine how cannabis use is related to cognitive functioning, and we hope that our study can help inform future study designs," Ellingson noted. "These studies are particularly important because cannabis is becoming more potent and more accessible as states legalise its recreational use."
* Christopher Bergland is an endurance athlete, coach, author and political activist.
Familial factors may not explain the effect of moderate‐to‐heavy cannabis use on cognitive functioning in adolescents: a sibling‐comparison study
The journal Addiction. Published on 3 September 2020.
Authors
Jarrod M Ellingson, J Megan Ross, Evan Winiger, Michael C Stallings, Robin P Corley, Naomi P Friedman, John K Hewitt, Susan F Tapert, Sandra A Brown, Tamara L Wall and Christian J Hopfer.
Abstract
Aims
To examine whether moderate adolescent cannabis use has neurocognitive effects that are unexplained by familial confounds, which prior family‐controlled studies may not have identified.
Design
A quasi‐experimental, sibling‐comparison design was applied to a prospective, observational study of adolescents with moderate cannabis use. Participants were recruited from 2001 to 2006 (mean age = 17 years). A second wave of data was collected from 2008 to 2013 (mean age = 24 years).
Setting and participants
Two US metropolitan communities. A total of 1,192 adolescents from 596 families participated in this study. Participants were primarily male (64%) and racially and ethnically diverse (non‐Hispanic white = 45%). A sibling in each family was a clinical proband identified due to delinquent behaviors.
Whereas prior family‐controlled studies have used samples of primarily infrequent cannabis users (mean = 1–2 days/month), participants here endorsed levels of cannabis use comparable to findings from epidemiological cohort studies (mean = 7-9 days/month).
Measurements
Semi‐structured clinical interviews assessed drug use, and a neuropsychological battery assessed cognitive abilities. Covariates included age at assessment, gender and alcohol use.
Findings
After correcting for multiple testing, a greater frequency and earlier onset of regular cannabis use were associated with poorer cognitive performance, specifically on tests of verbal memory.
Further, after accounting for familial factors shared by siblings and alcohol use, poorer verbal memory performance was still associated with greater life‐time frequency of cannabis use at wave 1 [b = −0.007 (−0.002, −0.012), adjusted P = 0.036]; earlier cannabis use at wave 2 [b = −0.12 (−0.05, −0.19), adjusted P = 0.006; b = −0.14 (−0.06, −0.23), adjusted P = 0.006]; and greater frequency of past 6 months use at wave 2 [b = −0.02 (−0.01, −0.03), adjusted P = 0.002; b = −0.02 (−0.01, −0.03), adjusted P = 0.008].
Conclusions
Moderate adolescent cannabis use may have adverse effects on cognitive functioning, specifically verbal memory, that cannot be explained by familial factors.
[link url="https://www.psychologytoday.com/za/blog/the-athletes-way/202009/cannabis-use-has-debatable-impact-cognitive-functioning"]Cannabis use has a debatable impact on cognitive functioning[/link]
[link url="https://onlinelibrary.wiley.com/doi/abs/10.1111/add.15207"]Familial factors may not explain the effect of moderate‐to‐heavy cannabis use on cognitive functioning in adolescents: A sibling‐comparison study[/link]