Study Links Stronger Cannabis Products to Psychosis and Addiction Risks

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High-concentration tetrahydrocannabinol (THC) products carry significant risks for mental health, particularly when it comes to psychosis and cannabis use disorder, according to research published in Annals of Internal Medicine. Additionally, the evidence for benefits in anxiety and depression is limited and inconsistent.

“Historically, cannabis has been prohibited nationwide at the federal level and classified as a Schedule I drug under the Controlled Substances Act,” explained lead investigator Thanitsara Rittiphairoj, M.D., M.P.H., from the Department of International Health at Johns Hopkins Bloomberg School of Public Health. “As of 1 February 2025, a total of 39 states, 3 U.S. territories and D.C. permitted medical cannabis, whereas 24 states, 3 U.S. territories and D.C. had legalized recreational cannabis.”

As cannabis becomes more widely legalized, many products on the market now contain very high levels of THC — the main psychoactive component in cannabis. While THC levels ranged from 2% to 4% in previous decades, the average THC concentration is now 20% in the United States, with some vaping devices delivering concentrations as high as 70% to 90%.

The team of investigators performed a database search to identify nearly 100 studies (encompassing 221,097 participants) to better understand how these high-concentration THC products may affect mental health. The outcomes included were anxiety, depression, cannabis use disorder, schizophrenia or psychosis. The team was interested in both the first occurrence and the progression of these conditions.

High-concentration THC was defined as greater than 5 mg or greater than 10% THC per serving, or those labeled “shatter,” “high-potency concentrate” or “dab.”

Among the included studies, most were either observational (47%) or randomized controlled trials (42%), and were conducted between 1977 and 2023. Most of the research was performed in the U.S. (34%), followed by the U.K. (25%) and Canada (10%). A large percentage of studies (58%) were categorized as a high risk of bias, compared with only 5% rated at low risk.

The findings raise concerns:

• Psychosis and schizophrenia: Most studies found that high-THC products were linked to a higher risk of psychosis or schizophrenia.
• Cannabis use disorder (CUD): High-THC products were strongly associated with problematic use and addiction-like patterns.
• Anxiety and depression: Results were mixed. In people without medical conditions, high-THC products were often linked to worsening anxiety and depression. But in some therapeutic settings, about half of the studies suggested potential short-term benefits, especially for anxiety and depression — though some patients still experienced negative effects.

Investigators noted the moderate and high risk of bias of some studies (nearly all the studies categorized as low risk of bias showed unfavorable findings) as well as the limited evaluation of newer products with high THC concentration. Therefore, Investigators emphasize that more high-quality research is needed, especially on these newer products.

“The findings with regard to anxiety, depression, psychosis or schizophrenia and cannabis use disorder are concerning — even among otherwise healthy persons, potentially including those using the products for recreational purposes — albeit not definitive for causation and imply a need for precautionary guidance,” investigators concluded. “As to concerns generally about the high-concentration products in the marketplace, there are authoritative conclusions that the risk for adverse consequences increases with THC dose.”