Danish investigators found that the proportion of schizophrenia cases associated with problematic use of marijuana increased 3-4 fold over the past 20 years. In a nationwide study that included nearly every person born in Denmark before 2000, the number of schizophrenia cases linked to cannabis use disorder increased from 2% in 1995 to between 6-8% by 2016. Previous research found that risk of psychosis and schizophrenia increases as THC concentration increases, so it is important to also note that marijuana potency rose by approximately 200% since the 1990’s when average THC levels were ~4%..

Drug Free America Foundation epidemiologist Dr. Sharif Mohr commented, “Marijuana has long been suspected of being an important contributor to development of psychotic disorders like schizophrenia. The association between the two was observed as far back as the 1800’s in asylum patients in Bengal, India. We also know that with more potent forms of the drug and more frequent use, the risk of psychotic disorders is greatly increased. For example, in the 2019 study performed by DiForti and colleagues, frequent users of high-potency marijuana had 10 times the risk of developing psychotic disorders as non-users and roughly 3-4 times the risk as less frequent users or those who used less potent forms of the drug.”

Dr. Mohr went on to say, “This should serve as a wakeup call for lawmakers and public health officials in the US. As use of high-potency marijuana becomes increasingly normalized and prevalent through legalization, there will be a corresponding rise in schizophrenia cases. Unfortunately, we may not even detect it because in contrast to countries like Denmark, no one is systematically tracking incidence and prevalence of schizophrenia on a national scale in the US.”



  • Hjorthøj C, Posselt CM, Nordentoft M. Development Over Time of the Population-Attributable Risk Fraction for Cannabis Use Disorder in Schizophrenia in Denmark. JAMA Psychiatry. Published online July 21, 2021. doi:10.1001/jamapsychiatry.2021.1471
  • Stuyt E. The Problem with the Current High Potency THC Marijuana from the Perspective of an Addiction Psychiatrist. Mo Med. 2018;115(6):482-486.
  • Di Forti M, Quattrone D, Freeman TP, et al. The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. Lancet Psychiatry. 2019 May;6(5):427-436. doi: 10.1016/S2215-0366(19)30048-3. Epub 2019 Mar 19.