Marijuana’s usefulness in helping treat epilepsy and PTSD has helped rebrand the drug as one that can benefit human health. But a link between marijuana and psychosis, a mental disorder associated with severe delusions or hallucinations, still persists, and the research on it has been inconclusive so far. On Tuesday in The Lancet Psychiatry, researchers provide a new glimpse into how strong that connection really is using data on the marijuana habits of psychosis patients from across Europe and Brazil.

The evidence and arguments linking marijuana to psychosis is controversial. Previous scientific research has linked THC, marijuana’s psychoactive component, with psychosis in some patients — but not all of them. Why does weed seem to be linked to psychosis in some people and not others? Marta Di Forti, Ph.D., the lead author of the new Lancet paper, argues that the amount and potency of marijuana are important factors in understanding the drug’s connection to psychosis.

"This effect was greater in London and in Amsterdam, where high potency is more available.

In their analysis of the marijuana habits of psychosis patients across Italy, Spain, the UK, the Netherlands, France, and one site in Brazil, the authors show that one in every five psychosis cases in the study sample was linked to smoking daily, and that one in every ten new cases was linked to smoking marijuana with a THC concentration above 10 percent.

“We clearly showed that across the 11 study sites, those who use high potency on daily bases have over a 4 fold increase in the odds of having a psychotic disorder,” Di Forti tells Inverse. “This effect was greater in London and in Amsterdam, where high potency is more available.”

Smoking potent marijuana every day was associated with psychosis in this study GIPHY

Marijuana Potency and Psychosis Cases

In the study, Di Forti and her team tracked how often patients smoked marijuana and estimated the potency of the marijuana based on national data that reported “expected concentrations.” They classified marijuana as “highly potent” if it had a THC concentration of ten percent or more. Then, they compared the smoking habits of the psychosis patients against those of healthy controls from each area.

"Both measure how much of cannabis’s active ingredient, THC, reaches our brain.

The psychosis patients generally reported significantly more daily smoking and higher rates of potent use. After pooling and reanalyzing this data, Di Forti found her key statistic: Daily cannabis users had a 3.2 times greater chance of having psychosis than people who had never smoked. Those who smoked the “potent” strains of marijuana every day had 4.8 times greater odds of having psychosis.

Di Forti says that frequency and potency of marijuana are ultimately two metrics for measuring the same thing: getting THC into the brain.

“Both measure how much of cannabis’s active ingredient, THC, reaches our brain,” she explains. “It is a little bit like alcohol. You can reach the same number of units per week either by drinking spirits or lots of wine daily.”

In this case, you could think of smoking the potent strains of weed every day as taking daily THC shots. Doing that consistently, at least going by the results in this paper, seems to be associated with greater risks of psychosis.

What This Paper Can’t Prove:

In an companying commentary Suzanne Gage, Ph.D., a lecturer in psychology at the University of Liverpool, points out that Di Forti’s paper does not establish the causal link that some may hope it does. At its core, it is still an observational study, and though Gage admits that the authors did their best to account for potential confounders, she says there will always be some grey area.

"It’s very definitively hard to say ‘yes, cannabis causes psychosis.’

“There’s lots of this observational data that suggests this link, but it’s very definitively hard to say, ‘Yes, cannabis causes psychosis,’” she tells Inverse. “It’s definitely not the case that everyone who uses cannabis every day will develop psychosis, so it just makes the pattern quite complicated. People who choose to use high potency cannabis every single day are likely to be different from people who don’t in lots of ways other than just their cannabis use.”

Part of the reason the data to make a strong link are inadequate is because psychosis cases remain rare, despite marijuana use becoming increasingly common. To add to that, there’s still a lot we don’t understand about psychosis as well as all the different bioactive components of marijuana.

CBD, the non-psychoactive component in marijuana, has actually been shown to help treat psychosis and other psychiatric symptoms. And as far as psychosis goes, says Gage, other factors like childhood trauma or a family history of psychosis can affect who actually ends up with the condition.

Nevertheless, the new paper is valuable because it provides more evidence to support the idea that there is a pattern between marijuana use and psychosis — one that warrants further investigation. But the more we learn about all the different effects of marijuana, and the hundreds of different components, it’s becoming clear that we don’t know what kind of pattern it really is. As research on the mental health-marijuana connection continues, we could soon find out whether it’s part of a far more complicated picture.

Abstract

Cannabis use is associated with increased risk of later psychotic disorder but whether it affects incidence of the disorder remains unclear. We 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.