A sweeping new analysis has found that cannabis products with high levels of the psychoactive compound THC may significantly raise the risk of psychosis, schizophrenia, and addiction, heightening concerns among researchers and health officials as cannabis use rises worldwide.
The study, published in the Annals of Internal Medicine, reviewed 99 research papers conducted between 1977 and 2023, covering more than 220,000 people. It concluded that products containing higher concentrations of delta-9-tetrahydrocannabinol (THC) — the chemical responsible for the “high” associated with cannabis — are strongly linked to serious mental health risks.
In Europe, cannabis remains largely illegal, but the European Union Drugs Agency (EUDA) estimates that 4.3 million people use the drug daily or nearly every day. Consumption spans smoking herbal marijuana, eating edibles such as gummies, and vaping potent concentrates like hash oil, “shatter,” and “dabs.”
The potency of these products has increased sharply in recent years. In 2023, cannabis resin in Europe had an average THC concentration of 23%, compared with 11% for herbal cannabis, according to the EUDA. By comparison, researchers considered products with THC levels above 10% or 5 milligrams per serving to be “high-potency.”
The review found that people using such products faced an elevated risk of psychosis and schizophrenia within hours of consumption. Long-term use was also tied to cannabis use disorder — a form of addiction — as well as respiratory problems and dependency symptoms.
“There’s a real warning in these findings,” said Dr. Jonathan Samet, a co-author of the study and professor of epidemiology at the Colorado School of Public Health. “We really need to be carefully watching what the consequences of these products are.”
Findings for more common mental health conditions, such as anxiety and depression, were mixed. Some evidence suggested high-potency THC could ease symptoms in patients with cancer or neurological disorders, but other studies identified harmful effects. Overall, researchers cautioned that the benefits were limited and often tied to specific medical conditions.
The EUDA has long warned that frequent and heavy cannabis use can exacerbate psychiatric problems, increase dependence, and worsen lung health. The new findings, while not yet definitive, reinforce these concerns and could influence future regulations.
Samet noted that any attempt to regulate THC levels would face challenges, since consumers could simply adjust their dosage. “There’s not a simple solution when it comes to potency,” he said. Still, he argued that greater awareness is essential, especially for those with existing mental health vulnerabilities.
“People who say, ‘Well, I’m using THC, what about it?’ … need to know that there are risks,” Samet added.
