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Use of higher potency cannabis products was associated with a greater risk for psychosis and dependency, a systematic review of 20 studies found.

In findings from the U.K. GAP study, for example, users of high-potency cannabis were nearly three times more likely to experience first-time psychosis compared with those who had never used cannabis before (adjusted odds ratio [aOR] 2.91, 1.52-3.60), reported Kat Petrilli, MRes, of the University of Bath in England, and colleagues.

And daily use of high-potency cannabis was associated with a more-than five times greater risk of psychosis (5.40, 2.80-11.30), which was not the case with daily use of lower potency forms.

These findings are also "partially independent of the occurrence of childhood trauma, which is a common risk factor for the development of psychosis," Petrelli and co-authors noted in .

The review also showed that high-potency cannabis was associated with a higher risk of dependency when compared with lower-potency cannabis.

"The need to understand the association of cannabis potency with mental health outcomes is especially pressing because of international increases in cannabis potency and the availability of higher potency cannabis products, which have been particularly evident in new legal markets," the team wrote in what they believe to be the first systematic review of the association of cannabis potency and mental health and addiction.

"Policy makers should carefully consider cannabis potency when regulating cannabis in legal markets, such as through limits or taxes based on THC concentration," the researchers urged.

"High potency" is used to describe cannabis products with high percentages of Δ9-tetrahydrocannabinol (THC), the investigators noted. They added that products with varying percentages of THC can be available to consumers where cannabis is legal, and THC concentrations in cannabis are known to have increased globally in recent decades.

The study cited concentrations found in Washington state, with 20% THC available in flower products and more than 60% THC in concentrated extracts. While lower-potency products are also accessible, THC concentrations have been rising across the board for the last 10 years, the researchers noted.

They reviewed a total of 4,171 articles focusing on the links between cannabis and mental health, with 20 published studies qualifying for the analysis, involving 119,581 participants. Eight articles focused on anxiety, eight on psychosis, seven on depression, and six were on cannabis use disorder.

One online survey they analyzed also found that those using higher-potency herbal cannabis products saw an increased chance of lifetime psychosis diagnoses as opposed to low-potency users (OR 1.28, 95% CI 1.07-1.53). Not all cannabis products examined in the studies that were part of the review were associated with increased mental health risks -- for example, butane hash oil saw no increased association with psychosis.

A small Japanese study of 71 patients found that high-potency cannabis users were seven times more likely to develop dependency syndrome as a result of their use (OR 6.9, 95% CI 1.19-25.15). Another study saw that those using high-potency cannabis were more likely to experience problems stemming from their usage.

Among the studies reviewed, there were no conclusive findings regarding the impacts of high-potency cannabis use on anxiety and depression.

"Our findings suggest that people who use cannabis could reduce their risk of harm by using lower potency products," said co-author Tom P. Freeman, PhD, also of the University of Bath, . "In places where cannabis is legally sold, providing consumers with accurate information on product content and access to lower potency products could help people to use cannabis more safely."

Study limitations, the team noted, included that the 20 articles in the final analysis varied in terms of quality, and that there was a low number of studies that met the review criteria to begin with.

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