Skip to main content

Verified by Psychology Today


The Cannabis-Schizophrenia Link Grows Stronger

Another cautionary tale about the significant risks of marijuana use.

A 34-year-old male with cannabis use disorder (CUD) recently came into the addiction treatment center where I am the chief medical officer.

He reported having a full-blown psychotic incident before arriving and showed signs of schizophrenia during his time with us. Schizophrenia is a serious brain disorder that causes a person to think, feel, and behave abnormally, and can make daily functioning nearly impossible.

Three things to note about this situation with our resident: First, it’s rare for schizophrenia to show up in males in their 30s. The typical onset is late teens to early 20s for males, and late 20s to early 30s for females. Second, weed/marijuana/cannabis is indeed highly addictive for certain individuals, as it was for our resident.

Third, the episode points to the larger reality that cannabis is significantly more potent than it’s ever been. The result: We’re now in a whole new world regarding the psychological effects it can have on people.

Add the fact that cannabis is far more widely available than ever—as of January 1, 2024, it’s been legalized for recreational use in 24 states plus Washington, DC—and you see the scale of the growing challenge.

Not Your Parents' Marijuana

I warn people about the dangers of marijuana all the time, and in fact, I’ve posted about it on this site several times. The problem is that these kinds of warnings often get lost amid the avalanche of media and marketing messages that marijuana is safe, that it “cures” nearly everything, and that it won’t get you addicted.

Well, it can get you addicted. And if it does, it can wreak havoc on your life. Even if it doesn’t become an addiction, marijuana may trigger changes in the brain like it did with my patient. That can lead to serious problems like chronic psychosis and schizophrenia.

Marijuana is especially dangerous when used by people under age 25 whose brains are still developing. CUD can literally rewire the brains of young people, with disastrous long-term results.

Three facts about marijuana:

  1. In the 1990s, the average THC concentration of marijuana in the U.S. was about 4 percent. (THC, or Tetrahydrocannabinol, is the psychoactive part of the cannabis plant.) By 2018, average THC concentration had nearly quadrupled to more than 15 percent.
  2. Research has shown that people who start using marijuana before age 18 are four to seven times more likely to develop CUD.
  3. Marijuana has the potential to “switch on” certain genes that can lead to schizophrenia in young people. Males are at a higher risk for this than females, but it happens in both genders.

In a massive epidemiology study of 6.9 million Danes published in Psychological Medicine in May 2023, researchers found a clear link between cannabis use and schizophrenia onset. A key finding: Over the 50 years covered by the study, 30 percent of all schizophrenia diagnoses among the study cohort could have been prevented if men aged 21 to 30 had not developed CUD. (In other words, CUD often led to schizophrenia in those men.)

The researchers also found a clear correlation between the rising cannabis potency over time and the increasing rate of schizophrenia diagnoses. One snapshot: The potency of cannabis in Denmark rose from 13 percent THC on average in 2006 to 30 percent in 2016.

As the study authors put it, the increase in schizophrenia cases rose “completely in parallel” with the increasing potency of cannabis.

Words of Advice

Please consider the following, and share it with friends and loved ones:

  • Avoid marijuana until you’re an adult. Better still, hold off until age 25 or older, based on the damage it can do to the developing brain.
  • Avoid marijuana if you have a family history of schizophrenia or other mental illness.
  • Avoid marijuana if you currently have schizophrenia or any other mental illness.
  • Urge a loved one or friend who has schizophrenia to avoid marijuana.

We in the medical community must keep educating the public about the dangers of this drug. It is not harmless, and it can be addictive—especially in the potent form it often takes today. We need to keep getting the cautionary word out to counterbalance the overly positive hype that exists around marijuana. We need to share this information with family members, friends, and our patients who are using marijuana.

And lastly, we need to help those who are addicted to obtain treatment in a compassionate and nonjudgmental way. They are not criminals or bad people. Rather, in using marijuana, they are playing with fire regarding their mental health. Their risk of getting badly burned may be higher than they realize.


Hjorthøj C, Compton W, Starzer M, et al. Association between cannabis use disorder and schizophrenia stronger in young males than in females. Psychological Medicine. 2023.

National Institute on Drug Abuse. (2020). Is Marijuana Addictive?…

More from Lantie Elisabeth Jorandby M.D.
More from Psychology Today