Many countries have legalized or decriminalized cannabis use, leading to concerns that this might result in an increase in cannabis use and associated harm.

Indeed, when it comes to “medical marijuana” the litigation and legislative trend seems to focus on the question of “cannabis – yes or no, – without contemplating the related questions of “how long” and “what kind.”

As the following study points out, there are highly potent forms of cannabis available in pot shops where an injured worker might fill a prescription. The legislators and litigation outcomes make no reference whatsoever to potent and dangerous types of cannabis which may sit side by side with other types in a pot shop.

Currently, cross-sectional and prospective epidemiological studies as well as biological evidence support a causal link between cannabis use and psychotic disorder. Meta-analysis shows a dose-response association with the highest odds of psychotic disorder in those with the heaviest cannabis use.

Nevertheless, it is not clear whether, at a population level, patterns of cannabis use influence rates of psychotic disorder.

For that reason, researchers aimed to identify patterns of cannabis use with the strongest effect on odds of psychotic disorder across Europe and explore whether differences in such patterns contribute to variations in the incidence rates of psychotic disorder.

To accomplish this mission, they targeted 901 patients aged 18 – 64 years who presented to psychiatric services in 11 sites across Europe and Brazil with first-episode psychosis and recruited controls representative of the local populations. The results of the study were published online this month in the Lancet Psychiatry.

Differences in frequency of daily cannabis use and in use of high-potency cannabis contributed to the striking variation in the incidence of psychotic disorder across the 11 studied sites. Given the increasing availability of high-potency cannabis, and the researchers say “this has important implications for public health.”

Daily cannabis use was associated with increased odds of psychotic disorder compared with never users increasing to nearly five-times increased odds for daily use of high-potency types of cannabis.

Use of high-potency cannabis was a strong predictor of psychotic disorder in Amsterdam, London, and Paris where high-potency cannabis was widely available, by contrast with sites such as Palermo where this type was not yet available. In the Netherlands, the THC content reaches up to 67% in Nederhasj and 22% in Nederwiet; in London, skunk-like cannabis (average THC of 14%) represents 94% of the street market whereas in countries like Italy, France, and Spain, herbal types of cannabis with THC content of less than 10% were still commonly used.

Researchers concluded that their findings are consistent with previous epidemiological and experimental evidence suggesting that the use of cannabis with a high concentration of THC has more harmful effects on mental health than does use of weaker forms.

If high-potency cannabis were no longer available, 12·2% of cases of first-episode psychosis could be prevented across the 11 sites, rising to 30·3% in London and 50·3% in Amsterdam.