This story is over 5 years old.

Study Claims Marijuana Is Addictive, Damaging to the Brain, and Linked to Schizophrenia

An inventory of studies conducted over the past 20 years concluded that pot can be harmful, but some advocates are skeptical of the findings.
Photo via Flickr

The increasingly popular notion that marijuana is a relatively safe recreational drug was challenged this week by a new study that claims that the drug is addictive, damaging to the brain, and may cause psychiatric problems, including schizophrenia, in young people that smoke it.

The National Addiction Center at King's College, London published the claims in the academic journal Addiction. The study is the work of Wayne Hall, Director of the Center for Youth Substance Abuse Research at the University of Queensland. Hall, who has frequently spoken about the adverse effects of pot, is not basing his findings on new research or experiments, but rather an inventory of studies that have been conducted over the past 20 years.


Many of Hall's findings pointed toward correlations between marijuana use and "adverse effects," including addiction, brain impairment, low school attainment, harder drug use, and psychiatric disorders, particularly in adolescents. But, notably, he didn't find or identify many effects that were definitively caused by marijuana use.

Leading anti-marijuana academics are paid by painkiller drug companies. Read more here.

For example, Hall found a correlation between marijuana use and poor verbal learning and memory function, as well as "educational attainment" (how far a person advances in school), but the findings were not causal. In other words, it still isn't clear whether marijuana causes people to drop out of school, or whether people that are likely to drop out are also more likely to try marijuana. Some studies, though, showed a drop in IQ in users that started smoking in adolescence and continued through their 20s and 30s.

The claim that marijuana is a "gateway drug," was, again, found by Hall to be more of an issue of correlation: regular adolescent marijuana users are more likely to use other illicit drugs, but it remains unclear why. Young people who have smoked pot at an earlier age than their peers have earlier access to harder drugs than others, and "socially deviant" youngsters who are more likely to try heroin and cocaine anyway often start using cannabis earlier than their peers, he explained.


Similarly, Hall said psychotic disorders are more likely to occur in those who have smoked pot and higher in regular users, but widespread consumption of cannabis hasn't increased the prevalence of schizophrenia in any measurable way.

"Our best guess," Hall said, is that the "risk of developing psychosis doubles" from 7 in 1,000 people to 14 in 1,000 people. According to Hall, people who already have psychotic symptoms and use pot tend to have their first psychotic episode at an earlier age.

DEA accused of obstructing research on marijuana benefits. Read more here.

Advocates at the National Organization for the Reform of Marijuana Laws (NORML) dismissed the study's claims of linking marijuana use to addiction and schizophrenia. Allen St. Pierre, executive director of the group, criticized what he called "England's near-obsession with the idea that cannabis causes schizophrenia, even though there's not much data or human experience to support it."

"The idea that the Brits think that marijuana makes you bonkers is an overarching theme that bleeds over to other commonwealth countries like Australia and New Zealand because there is such a strong cultural linkage," he told VICE News.

St. Pierre suggested that, "humans have been using cannabis for thousands and thousands of years," and said, "one could argue there's been a co-evolution of humans and cannabis, that we have these THC receptors in our bodies regardless of where you live and whether you've used it or not."


"Schizophrenia has also been around a long time and thankfully regulated to a very small portion of society," he added.

St. Pierre acknowledged that individuals can develop dysfunctional relationships with marijuana, but said there are no physical withdrawals and cravings that go along with cocaine, barbiturates, alcohol, and other drugs.

Experts from the American medical establishment said Hall's findings sounded about right to them. The National Alliance on Mental Illness said in a statement that, "the overwhelming consensus from mental health professionals is that marijuana is not helpful—and potentially dangerous—for people with mental illness."

Fewer Colorado teens believe marijuana is a risky drug to use. Read more here.

Dr. Sharon Levy, the Chair of the Committee on Substance Abuse for the Adolescent Substance Abuse Program at Children's Hospital Boston, told VICE News that recent research suggests some causality between marijuana dosage and lower education attainment.

"There is no question that marijuana use tends to come first," Levy said. "It's the number one illicit drug tried by kids, and usually comes first. But that's not enough to say 'gateway.' That's availability and all kinds of cultural reasons. The question really is, is it because you buy drugs on the black market you're more likely to buy other drugs, or that marijuana is doing something to the brain? We can't rule these things out."


Levy, who is also a spokeswoman for the American Academy of Pediatrics, said there is a correlation between brain changes in adolescents and the use of marijuana and other drugs. She emphasized that young people who rationalize their marijuana use by saying it is infrequent may still be at risk of developmental impairment.

"Every kid who's building their case for marijuana use says, 'I know it's not good for my brain, or my bronchitis, whatever, but everyone has said that's with heavy marijuana use and I don't use heavily, so how much is too much?'" Levy said. "I get that question a lot. There's actually no known safe limit of marijuana use."

Hall did not return requests for comment on this story.

Follow Colleen Curry on Twitter: @currycolleen

Photo via Flickr