If you read this January 10 Wall Street Journal piece, you might conclude that a wave of cannabis-induced psychosis among teenagers is sweeping the country. The story highlights research on the correlation between cannabis use and conditions like schizophrenia and bipolar disorder. It suggests access to higher-potency cannabis is leading to ever-increasing rates of psychosis in teens. 

But the actual data paints a different picture. Population studies of over 63 million people show that rates of psychosis do not go up amid legalization. Teen use rates of marijuana are actually going down. And our best science suggests that genetic factors—rather than cannabis use—are to blame for the correlation between cannabis and psychosis. 

Can cannabis really induce life-long psychotic conditions—or is this yet another example of confusing correlation with causation, a common source of “reefer madness?”

Exploring cannabis-psychosis links

The research on cannabis and psychosis is nuanced, but it definitely shows a strong correlation—as opposed to a causation—between the two. For instance, the data is clear that acute cannabis exposure can cause you to see vivid colors, or mis-hear sounds, or get anxious. These experiences have been described as transitory “psychosis-like” episodes—particularly for occasional users. 

Confusing correlation with causation

With that in mind, it might not be surprising that when teens who use cannabis develop permanent psychosis-related conditions, their doctors and loved ones jump to the conclusion that cannabis caused the shift. There are numerous stories of teens who used cannabis and were subsequently diagnosed with these challenging, life-long conditions. Statistically, individuals who use cannabis are more likely to have bipolar disorder or schizophrenia than the general population. 

But scientists have long struggled with the question of whether cannabis use actually causes these conditions—or if a third factor drives both. Based on cases where these conditions emerge after cannabis use, researchers estimate that cannabis-induced psychosis occurs in .0027% of consumers. But even at this exceptionally rare rate, correlative studies can’t prove whether cannabis is the cause of psychosis or not. 

There are many ways to interpret the data that associate cannabis with psychosis. For example, individuals with psychosis conditions could be more likely to use cannabis in the first place, as a form of self-medication. If this were true, it would result in the same strong correlation.

Other correlates: Alcohol, tobacco

Cannabis certainly isn’t the only factor linked with psychosis, even though it receives more publicity. Studies have found many things correlate with higher risk of psychosis, including the use of any recreational substance, like alcohol and tobacco. These substances are used at similar rates by schizophrenics as cannabis, and show similar statistical increases in psychosis risk. No one is debating, however, whether we should prohibit these substances. 

Life stressors

Stress also frequently correlates to psychosis. The stress associated with an event like a teenager leaving home to begin college can trigger an initial psychotic episode.

One review study found that the strongest factors correlated with developing lasting psychosis include a) living in an urban area and b) being an ethnic minority. Since cannabis is often used to combat stress, it’s possible that people who are stressed out enough to develop psychosis are more likely to seek out cannabis.

Spice, K-2, and designer drugs 

And lastly, studies that associate cannabis with psychosis don’t specify which specific cannabinoids are at play. For example, businesses like head shops and truck stops sell unlicensed, unregulated—and often illegal—synthetic cannabinoids (THC-O, THC-P, HHC, etc.) that can be hundreds of times more powerful than THC alone. 

The use of dangerous synthetics like Spice prevails in states that lack access to legal botanical THC. Athletic and military THC testing regimes also induce players and servicemembers to seek out novel, dangerous synthetics that won’t show up in a drug test.

Data suggests that using synthetic cannabinoids can induce more symptoms of psychosis than natural cannabis.

Legalization isn’t increasing teen use

Some research also suggests that cannabis use among teenagers in particular has a strong correlation with a subsequent diagnosis of a psychosis-related condition. This could mean that teens are particularly vulnerable to cannabis’ effects. But it could also point towards the fact that conditions like bipolar disorder and schizophrenia are most commonly identified and diagnosed while patients are in their twenties, while cannabis use usually begins during teenage years. It is possible that individuals with these conditions would be more likely to self-medicate with cannabis; since this typically occurs before their diagnosis, doctors and scientists would assume that cannabis caused the psychosis-like condition.

While many prohibitionists would use any potential for teen psychosis as a reason to limit cannabis legalization, it’s important to note that rates of teen cannabis use are going down, not up, as states legalize cannabis. Street dealers do not check ID, but legitimate dispensaries do. This means that legalization tends to limit teen access, rather than increasing it. 

Psychosis rates *do not* increase with cannabis legalization 

Fortunately, scientists dig deeper than simple correlative studies. For instance, they look at the rates of psychosis in places where cannabis is more available. If cannabis’ prevalence causes psychosis, it would follow that as cannabis access increases, we would see rising rates of psychosis. 

While legalization does increase rates of other conditions that are directly triggered by cannabis use, such as cannabinoid hyperemesis syndrome, the studies that examine psychosis rates have had very mixed results. 

The WSJ article mentioned earlier in this article cites a small Danish study that examined data collected from 6,788 people in 11 locations throughout Europe and Brazil. None had access to tested, legal cannabis.

While it did find a link between access to high-potency cannabis and psychosis, a much larger US study involving over 63 million people found that access to medical or recreational cannabis had no impact on psychosis rates at all. If we were seeing a dramatic wave of teen psychosis in the US, as suggested in the recent WSJ piece, that shift should be reflected in this large-scale population study. 

Genetics may drive cannabis use as well as mental health outcomes 

Psychosis rates haven’t skyrocketed with the recent increase in legal cannabis access, so why do we see any correlation between the two? 

Beyond the possibilities suggested above, genetics may play a key role. Scientists have found data that points to underlying genes that predispose someone to both psychosis and cannabis-use disorder. (Doctors diagnose cannabis-use disorder when someone experiences negative effects from cannabis use, wants to stop using it, but cannot bring themselves to quit despite repeated efforts.)

Research reviews have found that an individual who has genetics correlated with cannabis-use disorder may be more likely to develop schizophrenia, whether or not they actually ever used cannabis. This suggests that genetics, rather than cannabis, is to blame. 

One the best-designed studies just came out. In January, researchers published a rare study of more than 4,000 identical twins over time. One twin used weed while the other didn’t, and researchers tracked them for 27 years. Weed use did not predict mental health outcomes.

“Cannabis consumption did not predict within-pair differences in psychoticism,” the researchers concluded.

There’s a big takeaway here: If you have a family history of mental illness, and are increasing your cannabis use, you might want to re-evaluate your habits.

The data also supports maintaining 21 as the legal appropriate age for cannabis consumption; that’s typically towards the back end of when congenital mental illness may crop up in an individual. 

While more research may shift our understanding of these issues, the current data does not suggest that cannabis use is a strong causal factor for chronic mental illness. Still, given cannabis’ ability to cause visual effects or strange thoughts, THC may be best avoided by at-risk patients. At the same time, doctors are actively evaluating whether the cannabinoid CBD can treat mental illnesses. 

As always, consult with a cannabis-specializing doctor before beginning medical use of cannabis to treat your symptoms.