A comprehensive review published in The BMJ has revealed that cannabis use may be especially harmful during adolescence, among those with mental health disorders, during pregnancy, and while driving.
On the other hand, the review also highlights the efficacy of cannabidiol, a compound in cannabis, in treating epilepsy and the usefulness of cannabis-based medicines for multiple sclerosis, chronic pain, inflammatory bowel disease, and palliative care.
The recommendations are based on an “umbrella review” of 101 meta-analyses on cannabis and health, synthesizing previous meta-analyses and providing a high-level summary of evidence on a particular topic.
The researchers noted that an increasing number of studies have examined the effects of cannabinoids on health and other outcomes, but most findings are observational and prone to bias, making it difficult to draw firm conclusions.
To address this, the international team assessed the credibility and certainty of over 500 associations reported between cannabis and health in 50 meta-analyses of observational studies and 51 meta-analyses of randomised controlled trials, pooling data from hundreds of individual studies published from 2002 to 2022.
The evidence was graded as high, moderate, low, or critically low certainty in randomized trials, and as convincing, highly suggestive, suggestive, weak, or not significant in observational studies, based on quantitative criteria.
The researchers found an increased risk of psychosis associated with cannabinoids in the general population based on at least suggestive level evidence in observational studies and moderate certainty evidence in trials.
Specifically, cannabis use was associated with psychosis in adolescents (when brain development is still taking place), and with psychosis relapse in people with a psychotic disorder, the study revealed. This suggests that cannabis use increases the risk of the onset of psychotic disorders and worsens clinical outcomes after its onset.
The study also found an association between cannabis use and general psychiatric symptoms, including depression and mania, as well as detrimental effects on memory, verbal, and visual recall based on weak to suggestive observational evidence and high to moderate certainty trial evidence.
These associations are particularly concerning given the peak age of onset of mental health disorders coincides with the age pattern of cannabis use disorders, from around mid-teens to early 20s, when adolescents and young adults are still in education.
Weak to convincing level observational evidence suggested a link between cannabis use and motor vehicle accidents. Convincing observational level evidence also linked cannabis use during pregnancy to the risk of having a small, low birth weight baby.
However, the study found cannabidiol to be beneficial in reducing seizures in certain types of epilepsy, while cannabis-based medicines were beneficial for pain and muscle stiffness (spasticity) in multiple sclerosis, chronic pain in various conditions, and palliative care, although not without adverse events.
The umbrella review is the first to pool observational and interventional studies on the effects of cannabinoids on humans. However, the researchers noted that most outcomes associated with cannabis use are supported by weak evidence, have low to very low certainty, or are not significant.
Other limitations include differences in the cannabis content of products, the fact that not all individuals will experience the same effects of cannabis use on their mental health and cognition, and that randomised trials might not be representative of the real-world population.
Nevertheless, the researchers recommend that law and public health policymakers and researchers should consider this evidence synthesis when making policy decisions on cannabinoids use regulation, and when planning a future epidemiological or experimental research agenda.
The experts also call for future guidelines to translate current findings into clinical practice, involving stakeholders in the process.
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