Folie cannabis ?

LONDRES – Le cannabis est-il dangereux pour la santé mentale ? Cette question a provoqué de nombreux débats au fil des années, suscitant souvent plus de passion que de raison. Conclusion : le consensus au sein de la communauté scientifique est que la plupart des individus qui font usage du cannabis ne développeront pas de problèmes mentaux. Certains sont cependant plus susceptibles d’en subir les effets négatifs.

On a longtemps pensé que le cannabis était une drogue relativement sans danger, et que les inquiétudes à propos de son utilisation étaient exagérées. Certains psychiatres avaient conclu qu’un usage excessif pouvait entrainer un état psychotique, avec hallucinations, délires et pensées perturbantes. Mais la première preuve significative d’un lien entre l’usage de cannabis et la maladie psychotique date de 1987, établie dans le cadre d’une importante étude Suédoise réalisée sur plus de 50 000 sujets suivis pendant 15 ans. L’usage déclaré de cannabis au début de l’étude augmentait la probabilité d’un diagnostique de schizophrénie dans les 15 ans à suivre. Plus l’utilisation est importante, plus la probabilité d’un tel diagnostique est forte.

Curieusement, cette découverte n’a guère soulevé d’intérêt, et aucune étude du même type n’a été réalisée jusqu’en 2002. Depuis, cependant, de nombreuses études ont exploré l’association entre cannabis et désordre psychotique. En 2007, une compilation des meilleures études avait conclu qu’un usage fréquent (quotidien) de cannabis doublait le risque de désordre psychotique. Dans la mesure où la prévalence au cours de la vie d’une maladie psychotique concerne 1% de la population, avec un usage quotidien du cannabis, ce chiffre passerait à 2%.

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