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High-potency cannabis linked to increased risk of psychosis and addiction, study suggests

The potency of marijuana has been rising for nearly half a century.
A person rolls a joint during the Mile High 420 Festival in Denver on April 20, 2022.
A person rolls a joint during the Mile High 420 Festival in Denver on April 20.Patrick T. Fallon / AFP via Getty Images file

The high-potency cannabis that is now widely available may raise the risk of both psychosis and addiction, a report published Monday in The Lancet Psychiatry finds.

The potency of cannabis — measured by how much THC is found in the product — has been rising for nearly half a century, increasing by about 0.29% every year from 1970 to 2017, according to earlier research. THC is the chemical in cannabis responsible for its psychoactive effects.

That change could have important implications for public health, experts say.

“With the increasing strength of cannabis available in the U.S. and around the globe, it’s important to understand the long-term health outcomes that might be associated with using these types of products as compared to what has traditionally been available,” said Ziva Cooper, the director of the UCLA Center for Cannabis and Cannabinoids.

Marijuana is legal for recreational use in 19 states and Washington, D.C., according to the National Conference of State Legislatures. Vermont is the only state that caps potency for cannabis products, although other states are looking into adding limits.

The finding that high-potency cannabis is associated with a greater risk of psychosis and addiction “definitely should give people pause,” said Cooper, who was not involved with the new research.

In Monday’s study, researchers analyzed data from 20 studies that included a total of nearly 120,000 cannabis users. They found that people with first episodes of cannabis-related psychosis were much more likely to have been using products with high levels of THC than products with low levels of the intoxicating chemical. Those using high-potency weed were also much more likely to become addicted, compared to those who used low-potency cannabis.

“We know from animal studies that higher doses of THC are more likely to cause addiction than lower doses,” said the study’s lead author, Kat Petrilli, a doctoral student in the department of psychology at the University of Bath in the United Kingdom. “As people are exposed to higher doses of THC through the use of higher-potency cannabis, this may increase the likelihood that it can cause long-term changes, such as to cannabinoid receptors in the brain which are associated with psychosis risk.”

In addition to psychosis, the studies included in the analysis focused on the impact of high-potency cannabis on several other aspects of mental health, including anxiety, depression and addiction.

The studies consistently showed an increased risk of psychosis among those using high-potency cannabis compared to those using low-potency products, with one report finding a nearly sevenfold rise in risk. When it came to addiction, the research showed an increased risk in those using high-potency products compared to low-potency cannabis, with one report also revealing a nearly sevenfold increase in risk.

The studies were mixed when it came to high-potency cannabis and anxiety and depression.

The fact that the researchers were able to find only 20 studies that met the criteria for inclusion in their analysis underscores how little research exists on the topic, Cooper said. The study still leaves many questions to be answered, she said, adding that the field is still “in its infancy.”

One issue is that the studies included in the new report had data only about whether people were using cannabis monthly, weekly or daily, she said.

It is important to look more closely at the daily users and to ask how many times a day they are using, Cooper said. “I think absolute THC exposure will be a really important variable as we start thinking more deeply about this,” she said.

Petrilli said, “Choosing to use lower-potency cannabis products could help to make cannabis use safer and offers a sensible strategy for harm reduction.”

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