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Review shows that cannabis use is a risk factor for schizophrenia

BMJ 2003; 327 doi: http://dx.doi.org/10.1136/bmj.327.7423.1070-c (Published 06 November 2003) Cite this as: BMJ 2003;327:1070
  1. Tony Sheldon
  1. Utrecht

    Public health researchers in the Netherlands now believe that there is “converging evidence” to show that using cannabis is a risk factor for schizophrenia.

    Researchers from the Netherlands Institute of Mental Health and Addiction warn that cannabis approximately doubles the risk of schizophrenia and that the risk increases in proportion to the amount of the drug used.

    The researchers draw their conclusions from a review of five longitudinal studies recently published in four medical journals, including the British Medical Journal (Nederlands Tijdschrift voor Geneeskunde 2003;44:2178-83).

    Data from these studies, which were carried out in the Netherlands, Sweden, New Zealand, and Israel, made it possible to clarify the sequence of events over a long period of time. They discounted the possibility that schizophrenia increases subsequent use of cannabis, the “self medication” hypothesis.

    The researchers excluded people with a previous history of psychotic problems from the analysis, and they controlled for the effects of other confounders, such as the use of other drugs. Thus they were able to show the specific influence of cannabis.

    The Swedish and Dutch studies also showed that the amount of cannabis used was associated with the risk of schizophrenia, supporting the causal hypothesis.

    In the Netherlands five in 10 000 adults a year develop schizophrenia. The researchers say that a conservative estimate based on consistent results is that using cannabis doubles the risk of developing the disease.

    They conclude that although the risk may be numerically small it is “serious in clinical terms” and that ignoring the message of the recent research “is not an option.”

    According to the 2003 national drugs monitor, a systematic collection and analysis of data carried out by the institute, the number of cannabis users in the Netherlands increased from 326 000 in 1997 to 408 000 in 2001.

    By the age of 18 years half of Dutch men and a third of Dutch women have used cannabis at least once. Of the people who say that have used it in the past month, half say they have smoked more than five joints.

    The researchers accept that epidemiological cohort studies do not offer complete proof, as there may be other underlying social or biological factors.

    But a co-author, public health researcher Filip Smit, said that public health officials don't require the same degree of certainty that scientists do. He said, “From a public health perspective we do not want to take the risk, especially with something as serious as schizophrenia.” The institute says its conclusions have implications for health education, and it wants more preventive health projects, especially ones that are geared towards teenagers at school.

    The paper has initiated a debate in the Dutch media, where a spokesman for the institute said, “Soft drugs have lost their innocence.”