Thus, in the United States, 33 states allow the medical use of cannabis, while 10 states have approved its use both for medical and recreational purposes.
Several countries across Europe and South America have also decriminalized cannabis, meaning that while its use is still illegal in those regions, the penalties that users can incur have lessened.
However, as laws against cannabis become less stringent, and its use for medical purposes gains in popularity, researchers are beginning to ask more questions about the potential risks of cannabis use, and which users are most likely to experience negative health outcomes.
Some specialists are particularly worried about how cannabis might affect the brain. One study, for instance, found that potent cannabis, or "skunk-like cannabis," can cause damage to the brain's white matter, which is made up predominantly of axons — the links that allow brain cells to "communicate."
Moreover, researchers are concerned about a potential link between cannabis use and instances of psychosis, a condition which causes a person to become unable to distinguish between real and imaginary events.
In a new study, a team of investigators from King's College London in the United Kingdom — in collaboration with colleagues from other institutions — have been looking to confirm the presence of this link. The researchers have also sought to understand which cannabis users are most likely to experience an episode of psychosis.
Their findings — which feature in the journal The Lancet Psychiatry — indicate that there is a strong association between drug potency and frequency of use and the risk of psychosis.
The study premise
In the current study, the researchers looked at the data of participants from 11 different places around Europe and South America, namely London and Cambridge in the U.K., Amsterdam and Gouda and Voorhout in the Netherlands, Paris and Puy de Dôme in France, Madrid and Barcelona in Spain, Bologna and Palermo in Italy, and Ribeirão Preto in Brazil.
The team explains they chose to focus on cohorts from these cities because these are places where highly potent cannabis is available for sale. Amsterdam, for instance, is famous for its "coffeeshops," where people can buy and use marijuana.
London is also becoming a global player in the cannabis industry, and there are some media reports about the emergence of "coffeeshops" similar to the ones in Amsterdam.
The researchers who conducted the present study note that, in such cities, many people describe psychotic experiences in conjunction with potent cannabis use.
To estimate the prevalence of psychosis in the locations that they focused on, the investigators first identified all the first cases of psychotic episodes that local mental health services reported in the 2010–2015 period.
Then, for more accurate results, the team compared the situation of 901 individuals who had experienced first-time psychosis with that of 1,237 healthy controls.
To start, the investigators gathered relevant information, regarding the participants' history of cannabis use. Then, they assessed the levels of potency for the types of cannabis that different participants preferred.
To do this, they looked at available data on levels of delta-6-tetrahydrocannabinol (THC), a main active ingredient of cannabis. They considered types of cannabis with more than 10 percent THC as "highly potent," and those with less than 10 percent THC as of "low potency."
Frequency and potency tied to risk
The researchers found that participants who had reported a first-time psychotic episode were much more likely than the controls to use cannabis daily.
Specifically, 29.5 percent of participants who had experienced psychosis (or 266 out of 901 individuals) used cannabis daily, while only 6.8 percent (or 84 out of 1,237) of controls did so.
Moreover, those who experienced psychosis were also more likely to prefer high-potency cannabis than their healthy counterparts. All in all, 37.1 percent of the participants (or 334 out of 901 individuals) confirmed using potent cannabis, while 19.4 percent (or 240 out of 1,237) of controls reported the same preference.
After adjusting for potentially confounding factors, the researchers noted that across the 11 study locations, daily cannabis users were three times as likely as never-users to have a first-time episode of psychosis.
Those who used highly potent cannabis on a daily basis were five times more likely to experience psychosis.
In fact, the researchers linked an estimate one in five (or 20.4) new cases of psychosis to daily cannabis use, and one in 10 (12.2 percent) to daily use of highly potent cannabis.
Understanding risk is of 'vital importance'
In Amsterdam and London, specifically, there was a strong tie between the use of high-potency cannabis and the presence of psychosis. The researchers linked four in 10 (43.8 percent) new cases of psychosis in Amsterdam to daily cannabis consumption and five out of 10 (50.3 percent) of new cases with the use of highly potent versions of the drug.
In London, the researchers believe they could link 21.0 percent of new cases to daily use and 30.3 percent to a preference for highly potent cannabis.
"Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms," notes lead study author Dr. Marta Di Forti.
"They also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level," she adds.