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Cannabis products with high concentrations of delta-9-tetrahydrocannabinol (THC) are associated with users developing cannabis use disorder (CUD) and schizophrenia, according to a new systematic review published in Annals of Internal Medicine. 

Researchers reviewed the effects of highly concentrated products containing at least 5 mg THC or more than 10% of the chemical per serving. High-concentration THC products were positively associated with people developing psychosis or schizophrenia (70%) and CUD (75%) in studies that did not investigate the potential of the drug for treatment of medical conditions. 

“The findings are concerning and warrant a precautionary approach to using cannabis products, particularly for those with preexisting psychosis,” said Jonathan Samet, MD, professor of epidemiology and environmental and occupational health at the Colorado School of Public Health in Aurora, Colorado, and an author of the study. 

THC concentrations have risen in recent decades. These products come in the form of shatter or dabs, with up to 80% THC. The appearance of shatter is similar to peanut brittle, but it is translucent and made by using a solvent like butane to extract high-potency cannabinoids from the plant’s buds. Dabs are made using a similar method but are waxier in texture. Both are heated, and the vapor is inhaled, or pieces can be rolled into joints. 

“Because cannabis is a natural substance, some patients are under the impression that it can only be beneficial and devoid of adverse outcomes,” said Luis E. Rosas-Vidal, MD, PhD, assistant professor of psychiatry at the Feinberg School of Medicine at Northwestern University in Chicago, who was not associated with the study. “This research argues against that.” 

Samet and his colleagues reviewed the findings of 99 studies that resulted in multiple associations with various mental health disorders. Associations were categorized as favorable (beneficial effects on health), unfavorable (harmful effects on health), null (no difference), or unclear. They included randomized trials (42%), observational studies (47%), and interventional studies (11%).

Nearly two thirds (61%) of studies included participants aged 18-59, with 4% including adolescents. Ethnicity and gender information were not collected. 

High-potency THC products were associated with participants developing CUD in 75% of associations. The association with psychosis or schizophrenia was identified during the first 12 hours of use and was present at follow-up of up to 2 months. 

High concentrations of THC may acclimate users to higher and higher doses, leading to addiction, according to the US CDC. 

Samet and his colleagues found mixed results regarding anxiety and depression. In nontherapeutic studies not measuring effects of products on anxiety or pain, studies participants experienced worsening anxiety in 53% of the associations and depression in 41% of the associations. Healthy participants — those with no comorbid conditions — were the most prone to these effects. 

In therapeutic studies, patients experienced relief from anxiety and depression in half of the associations. Those with cancer and neurologic conditions experienced the most benefits for anxiety. 

“When patients ask about using THC for the management of anxiety, my typical answer is that there is no conclusive evidence to recommend its use,” Rosas-Vidal said. “Also, if it is acutely beneficial, we do not have good data on longer-term outcomes/sustained benefit, and there exists the possibility of worsening anxiety in the long term.” 

As of 2024, about 22% of American adults aged 12 or older used marijuana products, up from 19% in 2021. 

“With cannabis and related products becoming legalized, their recreational and potential therapeutic uses have become a frequent topic of discussion between providers and patients,” Rosas-Vidal said. “Having a good understanding of what is known is vital to provide sound advice. Unfortunately, for many of the questions that come up, we still do not have a conclusive answer, and as providers we need to be transparent about that.” 

The study was funded by the Colorado General Assembly. Samet and Rosas-Vidal reported having no relevant financial disclosures. Additional study authors reported having no disclosures.

Brittany Vargas is a journalist who covers medicine, mental health, and wellness.