Adolescent Marijuana Use Increases Risk of Schizophrenia

first posted 27 October 2010

It turns out that the oft-ridiculed film “Reefer Madness” may contain a grain of truth for a small subset of young marijuana users.  The original 1938 film was entitled “Tell Your Children”, and, indeed, that is exactly what researchers now recommend for kids who intend to use marijuana. New and ongoing research has shown conclusively that marijuana use in adolescence raises the risk of developing a psychotic illness like schizophrenia.  The earlier in life marijuana is used, the higher the risk. A New Zealand study of 1000 people followed from birth to age 26 found that 10% of those who used cannabis before age 15 became schizophrenic versus only 3% of people who rarely, if ever, tried marijuana before age 18.

The evidence for the cannabis-psychosis connection has been building in the medical literature for years. Two studies of siblings published this year (one in Australia, the other in the Netherlands and Belgium) provided the strongest data yet; by using full siblings brought up in the same home, these studies eliminated many of the confounding variables found in other studies.  Although researchers stop short of saying that marijuana causes psychotic illness, they are now certain that it is one of the risk factors contributing to these diseases.  Theresa Moore and colleagues wrote in a 2007 issue of the British medical journal, The Lancet: “there is now sufficient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life.”  Researchers from the UK and New Zealand comment “Policy makers and law makers should concentrate on delaying onset of cannabis use.”

Heavy users and those whose use extends for a long time also increase their risk of developing psychosis. People who have used cannabis 50 or more times are six times more likely to develop schizophrenia than are non-users.  The 2010 sibling study in Australia and another related study showed that those who used marijuana for six years or longer were at significantly higher risk of having psychotic symptoms or developing a psychotic illness than those who used cannabis for a shorter period of time.  This study found that, when two siblings both use cannabis, the one who has used it for the longest time is at higher risk of psychosis.

Young people who start life at higher risk of developing a psychotic illness are more sensitive to cannabis and its psychotic effects.  Who is at risk? a) children with mentally ill family members; b) those who have experienced prenatal or childhood trauma; c) anyone who has a temporary psychotic reaction (hallucinations, delusions, distorted thinking) when using marijuana; and d) people who live in cities.  The 2010 Holland-Belgium study of siblings showed that people with a genetic risk for psychosis are more sensitive to the psychosis-inducing effects of marijuana, and the combination of genetic risk and cannabis use can lead to psychotic illness. Finally, researchers have found that marijuana use in people already diagnosed with psychotic illness worsens their symptoms.

Some evidence shows that a first psychotic episode in users is associated with higher potency cannabis. The potency of marijuana (concentration of delta-9-THC) in the U.S. has increased over time.  It went from 1.5% in 1980, to about 3% in 1983-1992, to 4.5% in 1997, and to 9.6% in 2007.  

Past misinformation and scare tactics about marijuana led Baby Boomers and following generations to dismiss almost any negative news about the drug.  As parents, this group tended not to worry about marijuana use in their children.  Since it’s true that most people can use marijuana without serious consequences, why make such a big deal about the small number of young people who may be tipped toward schizophrenia by cannabis?  If you are the kid who starts hearing voices, whose thoughts become twisted and non-sensical, who believes you’re being followed by the police...if you are the parent who watches a bright, social child disappear into the darkness, whose hopeful dreams for your offspring’s future are crushed...if you are one of these people, the link between marijuana and psychosis matters.  

Family members attending a NAMI (National Alliance for the Mentally Ill) support group admitted to starting out fairly lackadaisical about their children using marijuana.  However, most of them now tie marijuana to the initial decline in mental health they saw in their kids.  Many of the NAMI parents suspected that their children were “self-medicating” with marijuana.  A 2010 British Journal of Pharmacology paper definitively states that there is “overwhelming” lack of evidence for the self-medication hypothesis.  While studies have shown that marijuana use is not a risk factor for having depression and anxiety, researchers cannot rule out the possibility that some youngsters may use marijuana to self-medicate for these illnesses.  

  

Millions of Americans use marijuana despite its illegal status: 14.4 million had used it in the month before being questioned in a 2007 survey.  Sixty-two percent of the 2.1 million new users each year are under 18.   A global 2008 study by the World Health Organization showed that the U.S. and New Zealand had the highest rates of kids under 15 using marijuana.  Interestingly, by 21 years, 54% of Americans had used marijuana while only 35% of people in the Netherlands, where cannabis is decriminalized, had. Wikipedia reports that the percentage of people who had used cannabis in the last year was twice as high (12.6%) in the U.S. as in the Netherlands (6.1%).  Overall, the U.S. has the highest rates of both legal and illegal drug use in the world.

Update from the scientific literature (Oct. 2017):  https://www.scientificamerican.com/article/link-between-adolescent-pot-smoking-and-psychosis-strengthens/

Another update (May 2018):  https://www.washingtonpost.com/news/posteverything/wp/2018/05/25/feature/legalizing-marijuana-is-fine-but-dont-ignore-the-science-on-its-dangers/?utm_term=.a06c48b750f3

Yet another update (Jan. 2019): https://www.nytimes.com/2019/01/17/health/cannabis-marijuana-schizophrenia.html