Major review confirms cannabis use associated with poor mental health

The study was conducted by an international team of academics said: “Cannabis should be avoided in adolescents and young adults." File picture

Cannabis use is associated with poor mental health and the ability to think, a major new review has found.

It said the drug should be “avoided” in adolescents and young adults and when driving or pregnant. Researchers in what is described as an “umbrella review” of some 100 studies also examined cannabis-based medicines and found that “most outcomes” claimed are supported by “weak evidence”.

However, they did conclude that these medicines are “effective” for epilepsy, notably in children, as well as for multiple sclerosis, chronic pain inflammatory bowel disease and palliative care.

The study was conducted by an international team of academics and was published in the British Medical Journal. It said law and health policymakers “should consider” the study’s findings.

The report comes against the background of a growing examination across the Americas and Europe of laws around drug possession, and cannabis possession in particular, as well as the regulation of cannabis-based medicines.

The Citizens’ Assembly on Drugs will this weekend examine the legal approach to drugs in Ireland and the criminal justice system. A number of speakers and organisations have been prominent in the debate over what approach to take with cannabis use and supply.

The cannabis study said there is “convincing or converging” evidence supporting the conclusion that cannabis use is “associated with poor mental health and cognition [thinking]".

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In addition, they found that use was associated with increased risk of car crashes and could have “detrimental effects” in pregnancies.

It said: “Cannabis should be avoided in adolescents and young adults (when neurodevelopment is still occurring), when most mental health disorders have onset and cognition is paramount for optimising academic performance and learning, as well as in pregnant women and drivers.” 

The study said converging evidence, from multiple studies, shows that cannabis use is associated with psychosis in adolescents as well as adults.

It said: “Use of cannabis when no psychotic disorder has already occurred increases the risk of its onset, and using cannabis after its onset, worsens clinical outcomes.” 

Cannabis-based medicines

In relation to cannabis-based medicines, typically cannabinoids, it said that “most outcomes” associated with them are supported by “weak evidence”.

But it did find that they are “beneficial for pain and spasticity” in multiple sclerosis and for chronic pain in various conditions and in palliative care — but “not without adverse” effects.

It said a “mismatch was evident” between laws for cannabis-based medicine use and alcohol consumption. It said disability caused by alcohol is “roughly 25 times higher” than that caused by cannabis.