Cannabis and Chronic Non-Schizophrenic Psychoses


On the basis of their studies, some researchers have maintained that cannabis smoking can also give rise to longer-lived psychoses, sometimes referred to as “cannabis psychoses”. The conditions in question have been psychoses whose clinical picture has differed in certain respects from that of schizophrenia.

Still, despite the fact that studies have been carried out which support this position, the overall impression is that there is not sufficient evidence to support the existence of this alleged side-effect of cannabis smoking. In other words: what appears to be a separate type of functional psychosis is probably schizophrenia.

A number of scientific studies have been carried out to answer the question of whether cannabis smoking can provoke long-lived, possibly chronic, psychoses. This, then, does not refer to a toxic psychosis which is maintained by means of continued cannabis smoking, but rather to psychotic conditions which remain after the toxic effect has ceased, often for a long period of time unless the condition is stopped through successful treatment. Simplifying slightly, it can be said that these reports and the discussions that they have given rise to have dealt with two different questions: a) Can cannabis smoking lead to a protracted psychotic condition which is not schizophrenia? and b) Can cannabis smoking cause schizophrenia?

Cannabis Smoking and Functional Non-Schizophrenic Psychosis

Here as elsewhere, there is a good deal of confusion as regards definitions. It is often difficult to understand what certain researchers consider themselves to have proved within the complex field of various kinds of psychosis and neighbouring conditions. Negrete () has made an attempt to systematise the different conceivable psychotic conditions (including delirium), but this systematisation is too rarely used. Hall et al. () make a thorough effort to bring order to the discussion. It is clear that some of the researchers who have talked about a short-lived cannabis-induced psychosis have been misunderstood and taken to have been claiming that there exists a functional and potentially (in the absence of treatment) protracted psychosis.

Thacore and Shukla () have made the clearest attempt to demonstrate the occurrence of a specific cannabis-provoked functional psychosis. The psychoses described by them occurred, without exception, following a very long period of cannabis abuse; there was no evidence of delirium; and the picture presented by these psychoses differed from that found with schizophrenic patients in that there were no “negative” symptoms and a much lower degree of disturbance to thought. The researchers maintained that they could thus distinguish a variety of psychosis which differed both from the toxic cannabis psychoses and from schizophrenia. For various reasons, however, the attitude generally adopted towards this interpretation and other similar studies has been that the evidence does not yet allow any definite conclusions to be drawn.

Tsuang et al. () – who, it should be added, studied patients with various types of drug abuse – were able to separate, in an interesting manner, the purely toxic psychoses from those where drugs had contributed to precipitating what had probably been a latent psychosis. One of the differences found between these two varieties of psychosis was that there were clear signs of a hereditary disposition towards (increased vulnerability to) schizophrenia whereas no such heredity could be observed for the toxic psychoses. Most of the evidence seems to suggest that the purportedly functional cannabis psychoses are in fact primary schizophrenic psychoses or schizophrenic psychoses precipitated by cannabis abuse (). One potentially misleading circumstance is that cannabis smoking appears to affect the clinical picture exhibited by these probably schizophrenic patients.

It is difficult to find studies from recent years that reinforce – in a similar way as in the case of schizophrenia – the hypothesis that there is a specific cannabis-provoked (non-schizophrenic) chronic functional psychosis.

To conclude, then, the question of whether or not there exists a functional, non-toxic and non-schizophrenic cannabis-induced illness has not yet been answered.


Selections from the book: “Adverse Health Consequences of Cannabis Use. A Survey of Scientific Studies Published up to and including the Autumn of 2003