High-Concentration Delta-9-Tetrahydrocannabinol Cannabis Products and Mental Health Outcomes : A Systematic Review.

High-concentration THC cannabis products are consistently linked to worse mental health outcomes, especially psychosis or schizophrenia and cannabis use disorder. Among 99 studies (over 221,000 participants), 70% of non-therapeutic studies found unfavorable associations with psychosis or schizophrenia, and 75% found unfavorable associations with cannabis use disorder. For anxiety and depression, about half of non-therapeutic studies reported unfavorable associations, particularly in healthy populations. Some therapeutic studies suggested benefits for anxiety (47%) and depression (48%), but these findings were low-quality and inconsistent. More than 95% of the studies had moderate or high risk of bias.

Study at a glance

Abstract

Rapid changes in the legalized cannabis market have led to the predominance of high-concentration delta-9-tetrahydrocannabinol (THC) cannabis products. To systematically review associations of high-concentration THC cannabis products with mental health outcomes. Ovid MEDLINE through May 2025; EMBASE, Allied and Complementary Medicine Database, Cochrane Library, Database of Abstracts of Reviews of Effects, CINAHL, and Toxicology Literature Online through August 2024. Two reviewers independently selected studies with high-concentration THC defined as greater than 5 mg or greater than 10% THC per serving or labeled as "high-potency concentrate," "shatter," or "dab." Outcomes included anxiety, depression, psychosis or schizophrenia, and cannabis use disorder (CUD). Results were categorized by association direction and by study characteristics. Therapeutic studies were defined by use of cannabis to treat medical conditions or symptoms. Ninety-nine studies (221 097 participants) were included: randomized trials (42%), observational studies (47%), and other interventional study designs (11%); more than 95% had moderate or high risk of bias. In studies not testing for therapeutic effects, high-concentration THC products showed consistent unfavorable associations with psychosis or schizophrenia (70%) and CUD (75%). No therapeutic studies reported favorable results for psychosis or schizophrenia. For anxiety and depression, 53% and 41% of nontherapeutic studies, respectively, reported unfavorable associations, especially among healthy populations. Among therapeutic studies, nearly half found benefits for anxiety (47%) and depression (48%), although some also found unfavorable associations (24% and 30%, respectively). Moderate and high risk of bias of individual studies and limited evaluation of contemporary products. High-concentration THC products are associated with unfavorable mental health outcomes, particularly for psychosis or schizophrenia and CUD. There was some low-quality evidence, inconsistent by population, for therapeutic benefits for anxiety and depression. Colorado General Assembly, House Bill 21-1317.