Daily cannabis use, especially of the high potency strains, is linked to an increased risk of developing psychosis, according to new research released Tuesday.
The European study, which looked at cannabis use in 11 major cities and Brazil, is the first to show the impact of marijuana use on rates of psychosis, a severe mental condition, in large populations. The link with psychosis was strongest in London and Amsterdam where high potency strains — marijuana which contains over 10 percent THC, the psychoactive component of the drug — are highest and most commonly available.
In Amsterdam, half of all new cases of psychosis were linked with high potency use; in London, one-third of new cases were linked with high potency use.
“Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC, also known as skunk-like cannabis, has more harmful effects on mental health than weaker forms,” said Dr. Marta Di Forti, lead author from the Institute of Psychiatry, Psychology, and Neuroscience at King’s College, in London. “For the first time we have consistent evidence that there is a dose-dependent relationship between cannabis use and induced psychosis at a population level. The more cannabis you consume the more likely you are to develop a psychotic disorder.”
The new study, published in The Lancet Psychiatry journal, looked at 901 individuals with first episode psychosis who used mental health services across Europe between 2010 and 2015. The researchers collected information about participants’ history of cannabis use and other recreational drugs. They used published data to estimate THC levels in the types of cannabis used by participants.
The strains were classified as either high potency — over 10 percent THC — or low potency — under 10 percent THC.
Across the 12 sites, people who used any type of cannabis on a daily basis were three times more likely to have a diagnosis of a new episode of psychosis, compared with people who had never used cannabis, the researchers concluded. This increased to five times more likely for daily use of high potency cannabis.
Of all new cases of psychosis across the 12 sites during that time period, an estimated one in five new cases of psychosis were linked to daily cannabis use. One in 10 were linked to use of high potency cannabis, the study found.
“THC is the culprit in the psychotic events,” said Di Forti.
In the Netherlands, the THC content reaches up to 67 percent in certain cities such as Nederhasj. In London, skunk-like cannabis, known for its strong stench, represents 94 percent of the street market and has an average THC of 14 percent.
Previous research has linked weed use and psychosis, but the studies haven't been in large enough populations to provide reliable results.
A Canadian study published in the Journal of Child Psychology and Psychiatry in 2017 showed a substantial increase in “psychotic-like experiences” in teenage users. The study also reported adverse effects on cognitive development and increased symptoms of depression.
Other studies show that chronic use may even interfere with normal development of the adolescent brain.
“The results of this study need to be taken seriously,” said Dr. Adrian James, Registrar at the Royal College of Psychiatrists, in London.
“Cannabis carries severe health risks and users have a higher chance of developing psychosis,” James said in a statement. “The risks are increased when the drug is high in potency, used by children and young people and when taken frequently.”
Many countries have legalized or decriminalized cannabis use, leading to concerns that this might result in an increase in cannabis use and associated harms.
Studies on marijuana cannot be performed in the U.S. because the drug — which is now legal by a doctor’s order in 33 states and recreationally in 11 states plus the District of Columbia — remains in the same class federally as heroin and LSD, a Schedule I drug, designated as having “a high potential for abuse” and “no currently accepted medical use.”
Public health advocates of U.S. cannabis research believe marijuana laws should have no bearing on a scientist’s ability to study the drug’s consequences on a population.
This makes European and Canadian research all the more important, especially as more potent marijuana strains enter the U.S. market. For example, one study found that the average amount of THC in U.S. marijuana samples rose from 4 percent in 1995 to over 12 percent in 2014. During that same time, cannabidiol, the non-psychoactive component of marijuana, fell from 0.28 percent to 0.15 percent. This shift in the ratio of THC to CBD has a pronounced effect on the drug's perceived potency.
“CBD, which we refer to as the 'good guy' component of cannabis, has no psycho-active component and has even been show to offset the psychoactive components of THC in experimental studies,” said Di Forti.
In the U.S. the average THC potency of the cannabis flower products sold in Washington’s state-licensed markets is over 20 percent, and the average potency of extract-based products — like oils for vaping pens, and dabs — is in the neighborhood of 70 percent, Jonathan Caulkins, a drug policy researcher and professor at Carnegie Mellon University told NBC News.
Di Forti cautions that until more research is done, it is in most people’s best interest to stay away from high potency cannabis.
“If you want to experiment with it, do it rarely,” she said.