But the number of Americans who use cannabis heavily is soaring. In 2006, about 3 million Americans reported using the drug at least 300 times a year, the standard for daily use. By 2017, that number had increased to 8 million—approaching the 12 million Americans who drank every day. Put another way, only one in 15 drinkers consumed alcohol daily; about one in five marijuana users used cannabis that often. And they are consuming cannabis that is far more potent than ever before, as measured by the amount of THC it contains. THC, or delta-9-tetrahydrocannabinol, is the chemical responsible for the drug’s psychoactive effects. In the 1970s, most marijuana contained less than 2% THC. Today, marijuana routinely contains 20-25% THC, thanks to sophisticated farming and cloning techniques and to the demand of users to get a stronger high more quickly. In states where cannabis is legal, many users prefer extracts that are nearly pure THC.
In reality, accurately tracking psychosis cases is impossible in the U.S. The government carefully tracks diseases such as cancer with central registries, but no such system exists for schizophrenia or other severe mental illnesses. Some population-level data does exist, though. Research from Finland and Denmark, two countries that track mental illness more accurately, shows a significant increase in psychosis since 2000, following an increase in cannabis use. And last September, a large survey found a rise in serious mental illness in the U.S. too. In 2017, 7.5% of young adults met the criteria for serious mental illness, double the rate in 2008.
What is clear is that, in individual cases, marijuana can cause psychosis, and psychosis is a high risk factor for violence. What’s more, much of that violence occurs when psychotic people are using drugs. As long as people with schizophrenia are avoiding recreational drugs, they are only moderately more likely to become violent than healthy people. But when they use drugs, their risk of violence skyrockets. The drug they are most likely to use is cannabis.
The most obvious way that cannabis fuels violence in psychotic people is through its tendency to cause paranoia. Even marijuana advocates acknowledge that the drug can cause paranoia; the risk is so obvious that users joke about it, and dispensaries advertise certain strains as less likely to do so. But for people with psychotic disorders, paranoia can fuel extreme violence.
he first four states to legalize marijuana for recreational use were Colorado and Washington in 2014 and Alaska and Oregon in 2015. Combined, those four states had about 450 murders and 30,300 aggravated assaults in 2013. In 2017, they had almost 620 murders and 38,000 aggravated assaults—an increase far greater than the national average. Knowing exactly how much of that increase is related to cannabis is impossible without researching every crime. But for centuries, people all over the world have understood that cannabis causes mental illness and violence—just as they’ve known that opiates cause addiction and overdose. Hard data on the relationship between marijuana and madness dates back 150 years, to British asylum registers in India.
https://www.wsj.com/articles/marijuana- ... 1546527075 (paid subscription)
—Mr. Berenson is a former New York Times reporter and the author of 12 novels. This essay is adapted from his new book, “Tell Your Children: The Truth About Marijuana, Mental Illness and Violence,” which will be published by Free Press on Jan. 8.