The belief that weed kills brain cells has been long held by critics of the plant. Attempting to verify or refute these claims has been difficult  due to conflicting studies. Some research suggests that cannabis might be bad for adolescent brain and adult brains, but is the research substantiated? Has the research created a consensus in the scientific community about the affects of marijuana on the mind?

Does Cannabis Have Age-Dependent Effects? 

Although it is unclear if cannabis has neurotoxic effects on adult brains, there is a largely held belief that pediatric cannabis consumption can lead to long-term neurological harm. Claims that cannabis use is associated with lower cognitive functioning are largely based upon the findings of a single longitudinal study published by Meier et.al from Duke University.

In 2012, they reported that the onset of cannabis use in early adolescence was associated with an average decline of eight IQ points by middle-age. However, a 2016 critique of Meier’s study, published in the Proceedings of the National Academy of Sciences, stated that the reported differences in IQ were consistent with socioeconomic differences and likely were not attributable to marijuana use.

"[The] Duke team likely 'overestimate[d]' the impact of marijuana on IQ and they opined that the “true effect [of cannabis exposure] could be zero.”

Interestingly, in 2018, Meier and her colleagues themselves conceded “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."

According to the report "family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests."

It should be noted that a number of better designed and controlled studies have failed to replicate Meier’s findings. For example, a British study of more than 2,000 teens reported that cannabis exposure prior to the age of 15 “did not predict either lower teenage IQ scores or poorer educational performance […] once adjustment is made for potential confounds.”

What is the evidence with regards to the neurotoxic effects on brain development?

In 2017, researchers from the University of Colorado and Oregon Health & Science University, Portland, OR studied the brains of tobacco and cannabis users ( adults and teens) using MRI.

They reported, “neuroimaging studies provide strong evidence of deleterious effects of chronic alcohol use on brain structure in adults and adolescents,” [however] “no significant associations were observed between cannabis use and structural measures across any sample, even when limited to participants reporting weekly or greater use and removing the influence of the alcohol predictor.”

A 2019 European Archives of Psychiatry and Clinical Neuroscience review examined the most relevant existing literature on the age-related differences between the impact of cannabis on brain and cognition. The researchers were not able to determine that there is a causal effect between age of consumption and brain damage, but they did offer these three interesting hypotheses:

• Persistent cannabis using adolescents demonstrated worse executive function skills than adults age-effects are most apparent in heavy and dependent cannabis consumers.
• Adolescents are less likely to experience reduced cannabis cravings after intoxication than adults.
• Adolescents who abstain from cannabis for long periods of time are more likely to reduce the impact of neurotoxicity than adults.   

Avoiding cannabis before the age of 25.

While there may be a correlation between adolescent cannabis use and reduced cognitive function, there is no unequivocally established causal relationship. It remains unclear if low cognitive function leads to cannabis use or vice-versa. Despite these limitations, existing evidence is cautionary.

In ideal circumstances, cannabis consumption should not begin until brain development is complete, sometime around age 25. This is so not because of the alleged unproven neurotoxic effects but because we:

1. Don't understand the influence of the hundreds of chemicals that constitute cannabis that do cross the blood-brain barrier.
2. We do know that cannabis-use does have a mind-altering effect. 

In the case of the teen brain that is undergoing the second state of “pruning”  the first occurring before birth and so the question remains whether it is advisable to potentially influence this complex event by introducing hundreds of foreign chemicals from cannabis.

Common sense suggests that erring on the side of caution is the prudent thing to do and so cannabis use by teenagers appears to be inadvisable until we know more. So what about the use of cannabis as a therapeutic for pediatric disorders? As with so much in medicine, the choice is one of risk versus benefit.

Parents and guardians should always consult with their physicians before their child begins a cannabis therapy after weighing the benefits versus the potential risks that may arise through its use.

What About Long Term Use?

Recent studies have demonstrated that long-term exposure is not associated with changes in working memory. In a 2015 Canadian clinical trial assessing the safety of daily cannabis use showed no identifiable differences in neurocognitive skills compared to non-using controls.

In a 2019 study the authors from The University of Colorado stated that among subjects who had long-term cannabis exposure, 24 years of weekly use, “[did] not have a widespread impact on overall cortical volumes while controlling for age” despite over two decades of regular cannabis use on average.

The study also indicated that “no significant differences between [users and non-users] were observed in performance on a brief computerized cognitive battery.”

How Does Weed Work on a Human Brain?

Cannabis contains approximately 100 cannabinoids, or chemical compounds including tetrahydrocannabinol (THC) and cannabidiol (CBD). Cannabinoids produce a range of physiological effects after consumption because of the way that they interact with the endocannabinoid system (ECS), a neuromodulatory system responsible for regulating an array of functions including mood, appetite, pain sensation, pleasure and reward, reproduction, cognition, immunity, and sleep.

The ECS is comprised of cannabinoid receptors, many of which are concentrated in the brain.

Cannabinoids interact with these cannabinoid receptors in ways that signal the brain to produce varying effects. THC seems to have a strong binding affinity for CB1 receptors. CB1 receptors are densely located in areas of the brain that are associated with emotional, cognitive, and reward processing such as the frontal and limbic areas of the neocortex, hippocampus, amydala, cerebellum, thalamus, and basal ganglia. THC’s interactions with CB1 receptors are responsible for the cannabinoid’s psychoactive effects.

The benefits and risks of cannabinoid interactions with the ECS seem to be dose dependent. Cannabinoids have demonstrated medical efficacy in the treatment of a wide range of medical conditions including many seizure disorders and chronic pain. Patient experiences with cannabis have been so positive, that many substitute conventional pharmaceutical medications for pot. However, THC has been associated with adverse psychological responses when consumed in high doses.