The effect of regular cannabis usage on adolescent health and wellness is among the most deliberated and sensitive questions in cannabis. Beyond the matters of sociology, such as whether cannabis use leads to school drop-out or other drug use, adolescent cannabis use divides even the hard-science community. Science has, certainly, reached one major consensus: adolescent use of cannabis is not harmless. Below, Wikileaf explores the scientific dialogue to major modern health questions regarding adolescent cannabis use. Popular questions regarding adolescent use are its effect on 1. Psychosis 2. Cognitive Function and 3. Neurological Health.
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Psychosis

Cannabis has been proven to increase the risk of experiencing psychosis among certain vulnerable populations. About 3% of all people will experience psychosis in their lifetime. In a 2018 study, a Finnish scientist, Dr. Mustonen, found that 10.6% of 4511 adolescents practicing regular cannabis use reported an incidence of psychosis by age 30. Mustonen concluded that cannabis use hastens the onset and amplifies the symptoms of psychosis. A large body of studies corroborates that cannabis does, indeed, increase the incidence of psychosis among vulnerable adolescent users. Scientists agree that childhood trauma and family history of psychosis are implicated in the trigger risk. In a 2016 publication, Dr. Robin Murray found that the incidence of psychosis varies with the age of onset use and frequency of use.
The contentious matter is whether or not cannabis use causes psychosis in populations considered non-vulnerable: Will the adolescent with no associated childhood trauma or family history of mental disease experience psychosis due to heavy use of cannabis at a young age?
Dr. Marta di Forti, a colleague of the aforementioned Dr. Robin Murray at King’s College (London), set off a firestorm earlier in 2019 after claiming in a major publication that THC (specifically High-THC cannabis) causes psychosis. Di Forti’s claim upset the scientific community and drew a stinging reply from NORML. So many fellow researchers derided di Forti’s paper that Dr. Murray published an enjoinder addressing major criticisms.
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Despite the widespread rejection of di Forti’s claims regarding cannabis causing psychosis, it’s worth monitoring developments in follow up publications seeking to affirm or deny her conclusion.
psychosis iStock / andriano_cz

Cognitive Function

Scientists agree that adolescent cannabis use lowers performance across multiple cognitive performance measures. Scientists disagree over whether cognitive function rebounds to normal levels with abstinence or whether the vulnerable tissues in the adolescent brain sustain irreversible structural damage, compromising brain-function for life.
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The most influential publication in the field of cognitive performance among adolescent cannabis users has been the 2012 Dunedin study, which followed a cohort of 1,037 subjects from birth to age 38, charting, among many other things, the effect of cannabis use on the developing mind. The Dunedin study concluded that adolescent users experience deficiencies in executive function, processing speed, working memory, perceptual reasoning, and verbal comprehension. Cognitive dysfunction correlated with the regularity of cannabis use.
Despite the fame of the Dunedin study, large and small-scale research efforts have reached conclusions in conflict with its findings. Dr. Mokrysz analyzed data from 2235 teenagers monitored birth to age 16 in the Avon Longitudinal Study. Mokrysz found that the IQ and educational performance among adolescent cannabis users with 50+ uses at age 15 did not differ from their peers’ at age 16. Unfortunately, the study did not progress further. Mokrysz concluded that “cannabis use in teenagers may have less cognitive impact than epidemiological surveys of older cohorts have previously suggested.”
Over the last two years, the scientific community has increasingly supported the position that adolescent cannabis use causes no damage that’s irreversible. A 2017 longitudinal study of identical twins found little evidence that cannabis use was associated with IQ decline from ages 12–18. Lead scientist, Dr. Meier, concluded that “family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests.” A 2018 Harvard University study found that 30 days of abstinence reversed adolescents' exhibited deficiencies in learning, memory, and verbal learning (1 week), but not attention. There is not space enough, here, to fully explore the extent of scientists' ongoing disagreement concerning neurological health and adolescent cannabis use. The matter remains undecided, with good evidence on both sides.

Neurological Health

Pensive child school student with yellow lightbulb and school and childhood supplies design elements. Child ideas and development concept iStock / JNemchinova
Adolescent cannabis use strongly predicts negative mental outcomes occurring at least through mid-life. The 2015 Victorian Adolescent Health Cohort Study followed subjects from birth to age 35, examining cannabis use and mental outcomes. Researchers concluded that the use of cannabis before the age of 16 was the greatest indicator of later problematic mental health repercussions. 17% of occasional users and 45% of regular adolescent cannabis users experienced Cannabis Dependence Use Disorder at least once from age 20 to 35. The Cohort Study further found that all adolescent users showed a heightened expression of anxiety disorder, whether or not they abstained from cannabis after using as an adolescent.
Brain imaging and predictive modeling with machine learning in psychiatric neuro-imaging present scientists with new ways of identifying adolescents at-risk of adopting cannabis use. In 2017, Dr. Philip Aaron Spechler studied the brains of 1389 teenagers, seeking predictive identifiers. Spechler successfully identified behavioral and neurological commonalities among adolescent cannabis users. Common to both genders was greater lifetime alcohol and cigarette use by age 14, parental lifetime cannabis use, greater novelty-seeking personality traits, and a greater acceptance of risky behavior.
Male-specific predictors of adolescent cannabis use included muted or positive feelings towards deviant behavior, parental novelty-seeking personalities, and excitable personality traits. Escalation to regular use among adolescent males strongly correlated with perceived peer use. Brain imaging revealed that cannabis use by age 16 could be predicted by impaired processing of visual stop signal cues in the environment. More study is required to understand the neurological connection.
Female-specific predictors of adolescent cannabis use included extravagant personality in both the parent and daughter and greater impulsivity in parents. Escalation to regular cannabis use differed from males, as it correlated with high-risk alcohol use and antisocial behavior. Females using cannabis daily during adolescence were also more likely to report anxiety and depression symptoms at age 20. Brain imaging revealed that adolescent cannabis could be predicted by delayed maturation in the cortical cerebellum. This region of the brain is associated with inhibitory control as well as the processing of face stimuli. Spechler linked his findings with other studies’ conclusions that disfunction in the cortical cerebellum is associated with misperceived neutral faces as threatening as well as social anxiety disorder, another predictor of female cannabis use. The findings on the female brain function are extremely interesting and require further study.

Conclusion

Overall, science concerning the most sensitive issues facing parents and communities attempting to prevent and minimize cannabis use suggests that, while use is certainly risky, occasional adolescent cannabis use may not be as harmful to the mind as was once thought. That being said, we do not recommend underage consumption of cannabis unless recommended by a medical professional. Heavy and regular use during adolescence is particularly to be avoided and the onset of use should be delayed beyond age 16.
The Adolescent Brain Cognitive Development (ABCD) Study will be the next major publication in the field of adolescent cannabis use, following in the wake of the influential 2012 Dunedin study.  ABCD will subject children identified as high risk for early-onset substance use to regular MRI testing, seeking to uncover major revelations. ABCD has already begun producing regular research papers and will continue to do so, beyond the study’s termination in 2027.