Teenagers using cannabis increases the risk of schizophrenia 15 years later. And the increase is not related to the fact that these young people were already at an early stage of this disease, nor to the use of other substances such as tobacco, alcohol or other illicit drugs, nor to a family history of schizophrenia.
This is the conclusion reached by a team of Finnish researchers, whose work was recently published in April in the British Journal of Psychiatry.
The more cannabis you smoke, the greater the risk
It has already been shown that cannabis use increases the risk of delusional disorders, including schizophrenia. A meta-analysis published in 2015 establishes the link with a first psychotic episode and another published in 2016 with the risk of psychosis in general. Studies show that the onset of the disease, too, is earlier. Besides, there is a dose-response effect, i.e., the more you smoke, the higher the risk.
However, the available studies evaluate psychotic symptoms rather than a mental disorder characterized by such symptoms per se. These may be hallucinations (auditory, sensory, olfactory or visual), a delirium with a persecutory theme (the feeling of feeling threatened) or intuitive (being convinced, for example, that one is the hidden son of a historical figure). Another limitation is that these studies do not take into account the existence of prodromal (i.e., early) symptoms of schizophrenia in young people before cannabis use, or the parallel use of other psychoactive substances.
These two characteristics are confounding factors that may explain the observed association between cannabis use and the onset of schizophrenia.
A study of 6,500 Finnish adolescents using a vaporizer!
The cohort study conducted by the Finnish team goes further. It was conducted with 6,500 adolescents, followed from the age of 15-16 until their 30th birthday. This is an unusually large sample, given the difficulty of being able to reassess the same people 15 years later. Its size allows for robust results that are representative of the general population. They were using the test mighty vaporizer for this study, so they would not have to go through combustion!
These authors assessed the relationship between adolescent cannabis use and schizophrenia, taking into account the use of tobacco, alcohol and other illicit substances, a possible family history of schizophrenia, and the existence of prodromal symptoms in adolescence.
It is thus found that even taking these factors into account, the association between cannabis use and the subsequent risk of schizophrenia persists. The researchers conducted their evaluation on study participants at age 30 – the disease usually starts between the ages of 20 and 30. Thus, the risk of developing schizophrenia among participants between the ages of 15 and 30 years was significantly higher when they consumed five joints (cannabis smoked as a rolled cigarette), in all and for all, a level of use that can be considered quite common today.
Among adolescents with prodromal symptoms of schizophrenia, this risk was even higher (about twice as much).
Cannabis as a sufficient risk factor in itself?
Cannabis use in adolescence is, therefore, a subsequent risk factor for schizophrenia and delusional disorder. This over-risk is not explained by a family history of schizophrenia, the use of other psychoactive substances, or prodromal symptoms of schizophrenia.
Further studies will need to determine whether cannabis is a sufficient risk factor in itself, or whether this risk factor is in addition to a pre-existing vulnerability. In any case, these results argue for an approach to prevent and educate all adolescents about the risks associated with cannabis use at this pivotal time in the development of the brain.