The evidence linking cannabis use to psychotic disorders is considered strong enough to warrant a public health warning.
“Even as proponents of [cannabis] legalization contend that smoked marijuana is a harmless natural substance that improves the quality of life, a growing body of evidence links it in a small but significant number of users to…the induction or aggravation of psychosis.” “Psychotic disorders are arguably the most serious of mental illnesses, the best known being schizophrenia.” “Schizophrenia, an illness that is characterized by delusions, hallucinations, and odd behavior, is among the top 10 leading causes of disability in the United States. It affects approximately 1% of the general population.” Can cannabis cause it?
As I discuss in my video Does Marijuana Cause Schizophrenia?, over the last half-century, “nearly 2,000 studies have been published on this topic…and the pro-psychotic effects of cannabis have dominated media reporting about this drug. But how clear is the link?” Population studies have “consistently demonstrated a strong, positive, and dose-dependent association between cannabis use and the risk of psychotic disorders.” Indeed, studies have shown that the more cannabis people use, the more likely they are to be psychotic, as seen in a chart below and at 1:10 in my video. 
However, that doesn’t mean cannabis is the cause. It could just be a correlation or even a consequence of the disease. “The link between cannabis and psychosis is well established,” but it may be the case that patients with mental health problems self-medicate and “use cannabis to relieve their distress.”
As you can see below and at 1:38 in my video, there isn’t only a link between cannabis and psychosis in snapshot-in-time cross-sectional studies, but in cohort studies as well, where people are followed over time. Research has shown that cannabis use often precedes psychosis, not the other way around. Now, it goes without saying that “the vast majority of people who use cannabis do not develop psychotic disorders such as schizophrenia, and many people diagnosed with such disorders have never used cannabis.” But, overall, these studies are considered to be “strong enough evidence to warrant a public health message that cannabis use can increase the risk of psychotic disorders.”
There is another potential explanation: Even though cannabis use precedes schizophrenia, could it be that whichever genes drive schizophrenia also make it more likely you start smoking pot? The biggest strike against the cannabis-schizophrenia link is country-by-country ecological studies that don’t seem to show more disease in areas where there’s more use. And, overall, schizophrenia rates seem to have remained stable or even gone down worldwide since the 1960s, even though there’s been a big bump in cannabis use since then.
If about 10 percent of schizophrenia cases are attributable to marijuana use and there’s been a fourfold increase in use, why hasn’t there been a 40 percent increase in the prevalence of schizophrenia? The problem with that argument is “there is little reliable evidence on the temporal [true] trends in the incidence of schizophrenia, so it is difficult to know whether this statement is true or not.” Perhaps it’s more of an issue with potency rather than just cannabis in general. Indeed, “the incidence of schizophrenia is higher in countries…where high-potency cannabis has taken over the market compared with countries…where more traditional forms of cannabis are smoked.” The bottom line is you don’t know until you put it to the test.
You can’t just randomize kids to cannabis, but, in a way, Mother Nature set up a natural experiment for us. There are genes that kids randomly get that can increase their likelihood of smoking pot. Do those kids then go on to have a higher risk of schizophrenia? Yes, research “findings strongly support” all of those population studies that suggest “cannabis plays a causal [cause-and-effect] role in the development of schizophrenia.” Okay, but by how much?
Let’s break it down. Even if cannabis use doubles the risk, that would mean only going from a 7-in-1,000 chance of developing a psychosis to 14 in 1,000. So, going from a 1 in 140 chance to a 1 in 70 chance. It would be different if schizophrenia runs in your family, where a doubling of risk could mean going from a one-in-ten chance to one in five, but, on a population scale, it could take thousands of cannabis users quitting to prevent a single case of schizophrenia. So, from a public health standpoint, “addiction is a far more common problem.” Researchers “estimate that people who try cannabis are ninefold more likely to become addicted to it”—even though that itself is relatively rare—“than to develop psychosis in their lifetime.”
I have an entire series of videos on cannabis, which I originally released in a webinar and downloadable digital DVD. See related videos below.
