Teens and young adults who start using cannabis may have an increased risk of having psychotic experiences in the years following, a German study found.
Among young people who had never smoked pot and did not have any psychotic symptoms, those who started using the drug were nearly twice as likely to develop subclinical symptoms of psychosis, Dr. Jim van Os of Maastricht University Medical Center in the Netherlands and colleagues reported online in BMJ.
In a separate analysis, those who used cannabis consistently were more likely to report persistent psychotic experiences at more than one follow-up visit.
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The findings were independent of age, sex, socioeconomic status, use of other drugs, childhood trauma, and urban/rural environment, and remained significant after further adjustment for other psychiatric diagnoses.
There was no evidence that psychotic symptoms precipitated cannabis use, which would have suggested a self-medication explanation for the association.
According to the researchers, the results help clarify the temporal association between cannabis use and psychotic experiences, which are common and generally transitory phenomena that could potentially progress to a clinical psychotic disorder in the presence of certain environmental risks.
"Our study confirmed cannabis as an environmental risk factor, impacting on risk of psychosis by increasing the risk of incident psychotic experiences, and, if use continues over time, increasing the risk of persistent psychotic experiences," van Os and his colleagues wrote.
Although cannabis has been consistently associated with psychosis in prior studies, there is an ongoing debate about whether the relationship is causal, whether it can be explained by residual confounding, or whether it can be explained by the use of the drug to self-medicate for existing psychotic symptoms.
In an accompanying editorial, Dr. Wayne Hall of the University of Queensland in Herston, Australia, and Dr. Louisa Degenhardt of the Burnet Institute in Melbourne, Australia, argued that the study findings discounted the latter two possibilities, leaving only a causal explanation.
"Sensible reasoning supports the policy of providing young people with information about the risks of using cannabis," they wrote. "The case for communication is strengthened by evidence that regular cannabis use in adolescence predicts poorer educational outcomes, increased risk of using other illicit drugs, increased risk of depression, and poorer social relationships in early adulthood."
Self-Medication Ruled Out
Dr. Van Os and his colleagues examined data from the longitudinal, population-based EDSP (early developmental stages of psychopathology) study, which was conducted in the greater Munich area.
The current analysis included 1,923 individuals ages 14 to 24 at baseline in 1995. All completed follow-up visits at an average of 3.5 years and 8.4 years.
The presence of psychotic symptoms was measured using the Munich version of the composite international diagnostic interview (M-CIDI). Items addressing delusions, hallucinations, and passivity indicated the presence of subclinical psychotic experiences.
At baseline, 13 percent of the participants reported using cannabis at least five times at some point during their lives. That figure rose to 20 percent by the 3.5-year assessment.
The proportion of individuals overall who reported subclinical psychotic experiences was 23 percent at the 3.5-year visit and 12 percent at the 8.4-year visit. Rates were higher among those who reported using cannabis.
After adjusting for potential confounders, cannabis use was associated with both incident and persistent psychotic experiences.
Psychotic symptoms did not, however, precede cannabis use, which ruled out self-medication as an explanation for the association.
The researchers proposed that sensitization, in which repeated exposure to a stressor results in progressively greater responses over time, may explain the findings, "as our study showed that the risk of persistent psychotic experiences increases with longer periods of cannabis exposure."
They acknowledged some limitations, including the use of self-reported data, the lack of adjustment for a family history of psychosis, and possible bias from selected recall.