A major new study examining the association between potent marijuana, frequency of use and rates of psychosis has confidently revealed a link between stronger marijuana and higher incidences of psychotic disorders. The research suggests limiting the THC concentrations in cannabis may reduce adverse mental health issues associated with the drug.

There has long been a distinct correlation between marijuana use and the onset of mental illnesses such as schizophrenia. Although it still is unclear whether this association is causal, what is becoming increasingly clear is that concentrations of THC, the main psychoactive compound in marijuana, are rapidly increasing.

A striking report last year revealed the average THC concentration in herbal marijuana had doubled, from five percent to 10 percent, in the decade from 2006 to 2016. In some US regions where marijuana has been legalized, THC concentrations of up to 30 percent have been identified.

Perhaps even more concerning is the average drop in cannabidiol (CBD) levels seen in marijuana over the past decade. CBD is much less psychoactive than THC, and often pointed to as one of the more beneficial compounds found in marijuana. Most importantly, CBD has been found to be a potential antipsychotic agent, which hypothetically should balance out some of the negative effects caused by THC.

The new research tracked over 2,000 subjects across Europe, around half of whom appeared to suffer from a first-time psychotic episode between 2010 and 2015. Around 30 percent of those presenting with a first-episode of psychosis reported using marijuana daily.

The study broke down the marijuana users into two groups: a low potency group (marijuana with less than 10 percent THC), and a high potency group (more than 10 percent THC). While overall the study found daily marijuana use generally made a person three times more likely to suffer from a psychotic episode, this rate jumped to five times more likely when using high potency marijuana.

Putting these statistics in context, the researchers suggest the rates of psychotic incidences reported in Amsterdam, for example, would drop from 37.9 to 18.8 per 100,000 people per year if strong high-THC marijuana were not available.

Amir Englund, a researcher from King's College London who did not work on this study, suggests that while this study is impressive and important, it is still just an associational report, meaning causality is unclear.

"… studies such as these are unable to say whether it is the frequent use of stronger cannabis that is the cause of these patients having psychosis, but it also cannot be ruled out," says Englund. "It may be that cannabis causes psychosis for some, or that patients with psychosis prefer stronger cannabis, or both."

David Nutt, from Imperial College London, also suggests caution when determining causality from this kind of observational study. Nutt points out the psychosis group did report high rates of other drug use, including stimulants or hallucinogens, which can certainly be additional factors in the onset of an acute psychotic episode. Having said that, Nutt does add that THC, or more specifically d9THC, has been clinically found to trigger psychosis, so making low d9THC marijuana readily available for recreational uses may confer public health benefits.

"It is also important to realize that d9THC is well known to produce psychosis in healthy volunteers i.e. people without any predisposition to mental illness," says Nutt. "Indeed, it has been used for this purpose for many years and is one of the best models of psychosis we have."

The new research does offer an important insight into the potential negative mental health outcomes that could be associated with the increasing availability of high-potency strains of marijuana. As broader legal recreational acceptance of the drug seems to spreading across the world, this study suggests regulatory bodies need to pay attention to these new high-THC strains.

The research was published in the journal The Lancet Psychiatry.