"We have focused mainly on temporal order: Is it the chicken or the egg?" the study's lead author Merel Griffith-Lendering, a doctoral candidate in the Netherlands, wrote to Reuters. "As the study shows, it is a bidirectional relationship."
One huge flaw, however, in any theory which posits a causal relationship between teen cannabis use and the onset of psychosis is that statistically, psychotic symptoms have remained at the same low level of occurrence since before marijuana entered popular usage; in other words, psychosis is no more common today than it was in the 1940s or 1950s.
But some previous research had indicated an associated between teen marijuana use and the onset of psychosis. Scientists suggested that perhaps instead of marijuana causing mental illness, perhaps young people were using pot to ease their psychotic symptoms. (It's important to note that nobody is suggesting that adult marijuana use is associated with psychosis; no reputable studies exist to support that hypothesis.)
|Rochester General Health System|
|Dr. Gregory Seeger, Rochester General Hospital: "What is interesting in this study is that both processes are going on at the same time"|
On the other hand, a review article published in the British Journal of Psychiatry, published by the Royal College of Psychiatrists, strongly suggested that the odds of an associated between cannabis and psychosis are "low." A group of drug experts and psychiatrists, including scientists from the University of Bristol, Imperial College London, Cambridge University, and Cardiff University undertook to "systematically review the evidence pertaining to whether cannabis affects outcome of psychotic disorders."
The authors of that 2008 study concluded that -- despite prevailing clinical opinion -- it remains "unclear" whether cannabis led to worse outcomes for people with psychosis. "Confidence that most associations were specifically due to cannabis is low," they concluded.
"What is interesting in this study is that both processes are going on at the same time," said Dr. Gregory Seeger, medical director for addiction services at Rochester General Hospital in New York.
Scientists have seen as crucial the potential connection between teen marijuana use and risk of developing psychosis, particularly since young people's brains continue developing through their 20s.
The authors of the new study, published in the journal Addiction, point out that some earlier research has suggested that people who have existing, underlying psychosis may self-medicate by smoking cannabis in an effort to reduce symptoms or improve mood, reports Alice G. Walton at Forbes.
The study followed more than 2,100 Dutch teens, quizzing them about their mental health when they were 14, 16, and 19. The teens were asked about their cannabis use over the years and tested for "psychosis vulnerability," including questions about social relationships and feelings of loneliness, attention, and thought problems such as "seeing things that other people do not see."
Overall, the researchers found 940 teens -- about 44 percent -- reported smoking marijuana. Variables such as alcohol and tobacco use, as well as the mental health of parents and the socioeconomic status of families, were reportedly accounted for in the results.
The team said the marijuana-psychosis link works in both directions: Teens who smoked pot at 16 were more likely to have psychotic symptoms at 19, and teens who had psychotic symptoms at 16 were more likely to smoke pot at 19.
The researchers speculate that the "self-medication" hypothesis and the "brain damage" hypothesis both have some validity. For example, a person with psychotic symptoms might use cannabis to "improve their mood or to control one's feelings, boredom, social motives, improving sleep, anxiety and agitation." On the other hand, smoking marijuana could theoretically impact the developing brain, since the teen years are a "particularly vulnerable period for the effects of cannabis."
The current study, crucially, does not prove that marijuana causes psychosis. It only suggests that marijuana use and psychosis are connected, and that each may be a risk factor for the other. Dr. Marta Di Forti of Kings College, London, has also looked at the link between marijuana and psychosis in a separate study, and said she considers cannabis a risk factor for psychosis, but not a cause.
Prevention, the new study's authors said, should focus on addressing pre-psychotic symptoms in teenagers before they develop into full clinical psychosis.
Griffith-Lendering claimed she "could not say" how much more likely young cannabis users were to exhibit psychotic symptoms later on, when compared to non-users.