Cannabis should be avoided during adolescence and early adulthood, as well as by people prone to or with mental health disorders, according to an in-depth evidence review of cannabis and health.
The review, led by a team in Canada and involving researchers across the globe, also warned that it should also be avoided in pregnancy, and before and while driving.
However, writing in The BMJ today, they say cannabidiol is effective for people with epilepsy, and cannabis-based medicines can help people with multiple sclerosis, chronic pain, inflammatory bowel disease, and in palliative care.
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.
To address this, the team of researchers assessed the credibility and certainty of more than 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.
The studies, published from 2002 to 2022, looked at the effects of different combinations of cannabis, cannabinoids, and cannabis-based medicines on health.
The researchers graded the evidence as high, moderate, low, or critically low certainty in randomised trials, while in observational studies they graded the evidence as convincing, highly suggestive, suggestive, weak, or not significant, based on quantitative criteria.
Based on at least suggestive level evidence in observational studies and moderate certainty evidence in trials, the researchers found an increased risk of psychosis associated with cannabinoids in the general population.
Cannabis use was associated with psychosis in adolescents and with psychosis relapse in people with a psychotic disorder.
Based on weak to suggestive observational evidence and high to moderate certainty trial evidence, an association was found between cannabis and general psychiatric symptoms, including depression and mania, as well as detrimental effects on memory, verbal and visual recall.
The researchers say these associations are concerning given that the age pattern of cannabis use disorders coincides with the peak age at onset of mental health disorders, from mid-teens to early 20s.
Across different populations, weak to convincing level observational evidence suggested a link between cannabis use and motor vehicle accidents, while in pregnant women, there was also convincing observational level evidence for a link between cannabis use and risk of having a small, low birth weight baby.
Cannabidiol helped to reduced seizures in certain types of epilepsy, while cannabis-based medicines were beneficial for spasticity in multiple sclerosis, as well as for chronic pain and in palliative care, however not without adverse events.
Although the researchers say there are limitations to their study, they believe law and public health policy makers and researchers “should consider this evidence synthesis when making policy decisions on cannabinoids use regulation, and when planning a future epidemiological or experimental research agendas”.
Solmi M, De Toffol M, Kim JY et al. Balancing risks and benefits of cannabis use: umbrella review of meta-analyses of randomised controlled trials and observational studies. BMJ 31 August 2023; doi: 10.1136/bmj-2022-072348
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