"Individuals who use cannabis regularly have a 2- to 3-fold increased risk of a psychotic outcome," University of Bristol researchers wrote in the study. Past research
has calculated a statistical association between daily use of tobacco and an increased risk of psychosis.
For the new study, the research team looked at family data for 3,328 teens living in the Bristol area of the United Kingdom. The teens answered questions about their use of cigarettes and cannabis at six separate time points between the ages of 14 and 19. Since many people who smoke pot also smoke cigarettes, the researchers attempted to discover the separate health effects of each substance.
Analyzing the data, the researchers found a strong association between smoking cigarettes (only) at an early age and having a psychotic experience by age 18. These teens had a 4.3% higher probability of having a psychotic episode by age 18 as compared to teens who did not smoke. "Early" use or "late" use was not defined by an actual age.
The researchers also found that teens who only used cannabis at an early age experienced a 3.2% greater chance of having a psychotic experience as compared to non-users.
The most striking increased probability, though, occurred among teens who only used cannabis at a later age. They had 11.9% greater odds of psychotic experiences by age 18.
Next, the researchers looked at other factors in each teen's life -- alcohol use, bullying, social class and a family history of schizophrenia, among others -- to see if these might have swayed the results.
With these additional factors included in the analysis, the researchers found that the relationship between smoking cigarettes and psychotic-like symptoms weakened. However, the relationship between cannabis and psychosis remained strong.
The researchers also flipped their focus to see whether teens who had experienced psychosis during childhood showed increased cannabis or tobacco use. They found little evidence that early psychotic experiences led to increased use of either substance.
The study has a number of strengths according to Nehal Vadhan, assistant investigator at Zucker Hillside Hospital in Glen Oaks, New York. A psychologist, Vadhan was not involved in the research, though he recently published a different study
of marijuana use and psychosis.
Predisposition to psychosis
Strengths of the new study include a very large number of participants, "very rigorous" methods and measurements, a high proportion of female participants -- "which is not always common" -- and longitudinal measurement, where the researchers do not look at data from "a single slice in time but over a period of four or five years," said Vadhan.
Its weakness, according to Vadhan, is that the authors of the study didn't account for people who already showed a pre-disposition to psychotic-like experiences.
Psychologists classify some people as having "attenuated psychotic syndrome," a category for those at highest risk of an eventual psychotic break or illness. Such people will have symptoms "like visual illusions as opposed to hallucinations," explained Vadhan. For example, a visual illusion would mean seeing a shadow, say, and believing it was a frightening animal.
Another sign of attenuated psychotic syndrome is being prone to "magical thinking or overvalued ideation that don't quite reach the level of delusion," he said. Recent psychosocial decline -- poor interpersonal relationships, a drop in academic functioning -- adds more risk to the profile, he said.
People who fit this profile would be most at risk for cannabis triggering a psychotic disorder, he said, "because they're already partially the way there."
And so it is a weakness that the study authors did not account for all types of people who may be predisposed to psychotic experiences. Such people are "fairly common in the healthy population -- the people who aren't diagnosed with anything and never will be," said Vadhan. "Developing a psychotic disorder or developing symptoms that are just under the threshold for a psychotic disorder -- that's much more rare."
The researchers did include an important factor in their conclusion: the real-world behavior of marijuana users.
"Most cannabis users smoke cannabis in combination with tobacco," wrote the study authors. "Therefore, we cannot rule out whether the associations observed between the cannabis use class and psychotic experiences are exacerbated by the combined use of cannabis and cigarettes."