This page respects the original work's rights terms and independently introduces its bibliography, abstract, main results, and limitations. It is not a full translation.
About this resource
This page introduces a 2025 systematic review examining relationships between cannabis products with high concentrations of delta-9-tetrahydrocannabinol (THC) and mental-health outcomes. Its starting point is that concentrates, high-THC flower, and other products increasingly found in current markets should not be treated as identical to conventional lower-concentration cannabis. The review included 99 studies with a total of 221,097 participants.
The definition of “high concentration” varied among studies. THC percentage, product type, self-reported potency, and amounts administered in laboratory studies were mixed, so the review does not establish a single threshold. It covered psychotic symptoms, anxiety, depression, cannabis use disorder, and several other outcomes, organizing the direction of findings and certainty of evidence.
Study design
The authors searched for observational and intervention studies in adults and young people that explicitly identified exposure to high-concentration THC products. Comparators included lower-concentration products, nonuse, or different THC conditions. Risk of bias was assessed for each study, and more than 95% of the 99 studies were judged to have a moderate or high risk of bias.
This quality assessment is central to interpreting the conclusions. Even when an association is reported repeatedly, THC concentration alone cannot be established as the cause if pre-existing symptoms, other substances, frequency of use, CBD content, and social environment are not adequately adjusted. Product labels and self-reports also may not match actual chemical analyses.
Content
The review organized exposure to high-concentration THC products and outcomes involving psychosis or schizophrenia, psychosis-like experiences, anxiety, depression, cannabis use disorder, and other areas. Acute experimental studies and long-term observational studies have different meanings, and the review distinguishes short-lived symptom changes from the onset of chronic illness.
THC is the principal psychoactive constituent of cannabis, but actual products may also contain CBD, terpenes, and other cannabinoids. Extraction method and route of administration can change the speed and duration of exposure. The paper focuses not on whether cannabis as a whole is good or bad, but on how stronger THC exposure changes assessment of mental-health risk.
Results
The authors identified a consistent signal of concern linking high-concentration THC products with psychotic outcomes and cannabis use disorder. Some studies also reported associations with anxiety and depression, but direction and certainty differed by outcome and study design. People using higher-concentration products may also use more frequently or in greater amounts, and separating those influences completely is difficult.
The results do not mean every individual user will develop a psychiatric disorder. Nor do they prove that lower-concentration products are safe. The practical implication is that concentration needs to be included as an exposure measure at the population level and that clinical histories and public-health research are limited if they ask only whether someone has used cannabis.
Limitations
Most included evidence was observational, limiting causal inference. Definitions of high concentration, measurement methods, participant ages, follow-up periods, and mental-health measures differed, so the studies could not all be combined into one effect size. Markets change rapidly, and older product classifications may not apply directly to current concentrates.
Reverse causation is possible if people with symptoms select high-concentration products for self-management. Genetic and family risk, co-used substances, sleep, social stress, and other factors also require consideration. The review itself states that most of the evidence had a moderate to high risk of bias.
Safety
High-concentration THC may intensify acute anxiety, confusion, perceptual changes, and impairment of attention and motor function. Particularly careful evaluation is needed for young people, situations involving driving or machinery, and people with a history of or risk for psychotic symptoms or cannabis use disorder.
This page does not guide selection of concentration or methods of use. Nor does it recommend self-treatment of symptoms or changes to prescription medication. Research associations and individual diagnosis are distinct; when there is acute confusion, severe anxiety, or risk of harm to self or others, ordinary medical and emergency support takes priority.
Source and rights
Original source: Rittiphairoj PW, et al. High-Concentration Delta-9-Tetrahydrocannabinol Cannabis Products and Mental Health Outcomes: A Systematic Review. Annals of Internal Medicine. 2025. DOI: 10.7326/ANNALS-24-03819.
Respecting the publisher’s copyright, this page is a detailed introduction that independently summarizes methods, results, and limitations based on public bibliographic information and the abstract. It is not a full translation or republication of figures and tables.