Cannabis should be avoided during adolescence and early adulthood; in pregnancy, by people prone to mental health disorders and while driving, according to experts in a study published in The BMJ on Thursday.

Cannabis contains over 100 cannabinoids, of which tetrahydrocannabinol (THC) and cannabidiol (CBD) are the most clinically relevant. THC can induce a psychoactive “high” and can foster dependence, as well as other adverse psychiatric health effects.  Conversely, CBD has certain anti-psychotic and anti-anxiety properties and one purified form, Epidiolex®, has even been approved by the FDA as a medication for certain forms of epilepsy.

The BMJ study confirmed that CBD is indeed effective in helping people with epilepsy, while some cannabis-based products containing THC can help ease multiple sclerosis, chronic pain and inflammatory bowel disease in affected adults.

The researchers based their findings on 101 meta-analyses on cannabis and health conducted over 20 years (2002-2022), grading evidence as high, moderate, low, or critically low certainty in randomised trials – and as convincing, highly suggestive, suggestive, weak, or not significant in observational studies.

“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,” according to the BMJ in a media release.

“To address this, an international team of researchers set out to assess 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.”

Increased risk of psychosis

There was an increased risk of psychosis associated with THC-containing cannabis in the general population, particularly in adolescents, and with psychosis relapse in people with a psychotic disorder.  

The researchers also found an association between cannabis and depression and mania, as well as detrimental effects on memory, and verbal and visual recall.

Observational evidence suggested a link between cannabis use and motor vehicle accidents, while pregnant women who used cannabis use had an increased risk of having a small, low birth weight baby.

Cannabis-based medicines were, however,  beneficial for pain and muscle stiffness (spasticity) in multiple sclerosis but increased the risk of dizziness, dry mouth, nausea and drowsiness. 

For chronic pain, cannabis-based medicines reduced pain by 30%, but increased psychological distress. 

For cancer, some cannabinoids reduced sleep disruptions but resulted in increased gastrointestinal events.

Cannabidiol (CBD) was, on the other hand, beneficial in reducing seizures in certain types of epilepsy, particularly in children – but came with an increased risk of diarrhoea.

Weak evidence

This umbrella review is the first to pool observational and interventional studies on the effects of cannabinoids on humans, but the researchers note that most outcomes associated with cannabinoid use are supported by weak evidence, have low to very low certainty, or are not significant.

They also point to other limitations in the study, particularly the wide variations in the make up of cannabis products, such as the proportion of psychoactive THC. Additionally, not all individuals will experience the same effects of cannabis on their mental health and cognition.  And randomised trials might not be representative of the real-world population. 

Nevertheless, the researchers conclude 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.” 

Millions are addicted

According to the Global Burden of Disease 2019 study, around 24 million people worldwide are addicted to cannabis, particularly men and people in high-income countries. 

In Europe, over the past decade, self-reported use of cannabis within the past month has increased by almost 25% in people aged 15-34 years, and more than 80% in people aged 55-64 years.

The potency of cannabis has also increased in Europe, with the THC content associated with the ¨high¨ and subsequent adverse psychiatric increasing from 6.9% to 10.6% between 2010 and 2019.

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