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Cannabis law reform: experts cite possible public health consequences

With an increasingly liberal attitude globally towards cannabis legislation, there is a potential for public health consequences with regard to respiratory health, traffic-related injuries, and the mental health of vulnerable populations.

The global trend towards liberalising cannabis legislation comes amid recognition that previous restrictions caused social harm and impeded medical research on phytocannabinoids.

The liberal legislation and attitudes toward cannabis may create opportunities to harness the pharmacological benefits of cannabis for epilepsy, chronic pain, nausea and spasticity, among other medical conditions.

Nonetheless, write Jason Bantjes, Charles Parry and Bronwyn Myers in the South African Medical Journal, laws that potentially increase access to cannabis could have negative public health consequences too.

Rigorous debate about these public health concerns is needed as South Africa moves towards ratifying the Cannabis for Private Purposes Bill introduced in Parliament in October 2020. The Bill, currently before the Portfolio Committee on Justice and Correctional Services, provides for adults to possess, cultivate and process a prescribed quantity of cannabis plants, and to use cannabis, while also explicitly acknowledging the need to protect citizens from potential harms.

This prompts questions about who should be protected from cannabis harms.

Drawing on recent systematic reviews and expert consensus, we argue that adolescents, pregnant mothers, and foetuses are three groups vulnerable to the harmful effects of cannabis, requiring special consideration as noted in recent portfolio vommittee hearings.

Cannabis exposure and adolescent development

The lack of well-designed longitudinal studies assessing the impact of cannabis on adolescent health makes it impossible to conclude that adolescent cannabis use is safe. If anything, the evidence highlights safety concerns, with regular cannabis use during adolescence associated with persistent functional and structural neurological changes, and cognitive and emotional deficits.

Adolescents are more vulnerable than adults to the negative effects of cannabidiol and tetrahydrocannabinol, with regular exposure during adolescence associated with more severe, persistent negative outcomes. It causes structural, functional and histological alterations in the brain’s frontoparietal, frontolimbic, frontostriatal and cerebellar regions, and affects the endocannabinoid system, which is particularly susceptible to the harmful effects of cannabis as it undergoes developmental changes during adolescence.

Prolonged cannabis use during adolescence also disrupts the neuromaturation processes occurring during this period, with synaptic pruning and white matter development particularly affected.

Adolescent cannabis use is also linked to cognitive deficits; adolescents who use cannabis frequently demonstrate more severe executive dysfunction than their adult counterparts. Critically, it is not yet clear whether these effects are reversible, with some evidence suggesting that cannabis related neurocognitive impairments persist into adulthood, even after prolonged abstinence.

Furthermore, craving and disinhibition may be greater among adolescents compared with adults, potentially making adolescents more susceptible to substance use disorders. Although it is unclear how adolescent cannabis use affects the onset of schizophrenia, some studies indicate this as an area of concern and a reason to delay the age at which cannabis use is initiated. Adolescent cannabis use is, however, associated with an increased risk of psychosis in adulthood and other psychopathology, including elevated risk of depression, suicidal ideation and suicidal behaviour.

The harmful effects of adolescent cannabis use on mental health appear mediated by genetic and environmental factors, although it is unclear how risk factors interact to cause negative outcomes. Nonetheless, most clinical and preclinical data point to a correlation between adolescent cannabinoid exposure and persistent adverse neuropsychiatric outcomes in adulthood. It’s possible cannabis is not solely responsible for the observed long-lasting neuropsychiatric effects of adolescent cannabis use, however, it is a significant component of the risk profile.

Consequences of perinatal cannabis exposure

Ethical and methodological problems associated with administering cannabis to pregnant women have resulted in an over-reliance on animal models to make inferences about the harms of in utero exposure. Furthermore, many pregnant women using cannabis also have poor nutrition and prenatal care, making it difficult to tease out the effects of cannabis on foetal development from these other factors.

Nonetheless, evidence suggests cannabis use during pregnancy increases the risk of adverse outcomes for mother and child, to the extent that the American College of Obstetricians and Gynaecologists advised physicians to discourage use during preconception, pregnancy and lactation.

Prenatal cannabis exposure is associated with an increased risk of foetal growth retardation and childhood behavioural disturbances. Infants exposed to cannabis in utero tend to have lower birthweights and may need placement in the neonatal intensive care unit compared with infants without exposure.

Cannabis during pregnancy and breastfeeding alters the developmental trajectory of multiple brain regions and may result in lasting functional consequences, including impaired higher-order executive functioning (i.e. impulse control, visual memory and attention) and attention deficit hyperactivity disorder during childhood, affecting academic performance and social adjustment.

Furthermore, cannabis interferes with the endocannabinoid system (present from approximately day 16 of gestation), believed to be integral to foetal brain development, which could adversely affect foetal brain growth, including structural and functional neurodevelopment.

These observed neurocognitive and behavioural changes seem to persist throughout life. The mechanisms through which in utero cannabis exposure causes neurocognitive and behavioural changes are poorly understood, but it appears cannabis influences epigenetic regulation, potentially causing persistent foetal genetic changes. Cannabis may be toxic to human chromosomes, with chromosomal changes caused by in utero cannabis exposure contributing to the lower fecundity and higher miscarriage rates among women using cannabis.

The multi-dimensional chromosomal and genome toxicity of cannabis may also contribute to congenital anomalies and cancer in children.


Regarding cannabis use during adolescence and pregnancy, scientific research may not have kept pace with the speed at which laws are being liberalised. This is not to say cannabis legalisation should be resisted, but rather that the evidence suggests adolescent use be discouraged and that pregnant women advised to avoid cannabis use.

It remains to be seen whether the need for caution will be reflected in SA’s revised legal frameworks.

Jason Bantjes Alcohol, Tobacco and Other Drug Research Unit, South African Medical Research Council, Cape Town, South Africa; Institute for Life Course Health Research, Department of Global Health, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town

Bronwyn Myers Curtin enAble Institute, Curtin University, Perth, Australia; Alcohol, Tobacco and Other Drug Research Unit, South African Medical Research Council, Cape Town

Charles Parry Alcohol, Tobacco and Other Drug Research Unit, South African Medical Research Council, Cape Town; Department of Psychiatry, Faculty of Medicine and Health


SAMJ article – Liberalising cannabis legislation in South Africa: Potential public health consequences for adolescents and pregnant women (Republished under Creative Commons Licence)


See more from MedicalBrief archives:


Cannabis and brain health – American Heart Association scientific statement


Pregnancy – Cannabis use linked to autism; alcohol raises miscarriage risk


Novel study of high-potency cannabis shows some memory effects


Does cannabis really affect memory? Here’s what research says




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