"Reefer madness" hysteria began in the late 19th century with reports of the lethal and deleterious effects of the "killer weed". Equally absurdly, and far more recently, the term "killer weed" was bandied around the tabloids making reference to the new strain of cannabis, skunk weed. Fortunately, for the most part, increasing knowledge of cannabis has led most of us to concede that it is no killer! However, there have been persistent concerns that cannabis use may precipitate or exacerbate psychotic symptoms. The existence of a "cannabis psychosis" (i.e., a psychosis that would not occur in the absence of cannabis use, the symptoms of which are preceded by heavy cannabis use and cease on abstinence) is widely debated with mixed evidence.

A small number of controlled studies have been conducted over the last twenty years either comparing persons with "cannabis psychosis" with persons who are schizophrenic, or comparing psychoses in people with and without a history of cannabis use.

Thacore & Shulka, for example, compared 25 cases of "cannabis psychosis" with 25 cases of paranoid schizophrenia with no history of cannabis use. The cases had a paranoid psychosis resembling schizophrenia in which there was a clear temporal relationship between prolonged use of cannabis and the development of psychosis on more than two occasions. Patients with the "cannabis psychosis" exhibited more odd behaviour, violence, panic and insight, and less thought disorder than those with schizophrenia. They also responded quickly to anti-psychotics and recovered completely.

Many other studies have reported similar findings to claim relationships between the presence of cannabinoids in the urine and having a psychotic diagnosis. However, all of these studies can be criticised in terms of the fallability of clinical judgements about aetiology, the poorly specified criteria used in diagnosing these psychoses, the dearth of controlled studies and the striking variations in the clinical features of "cannabis psychosis". Further, there is a vast body of research, while conceding that cannabis use may exacerbate psychosis in a pre-existing disorder, which finds no evidence for the existence of a definable "cannabis psychosis".

Imade & Ebie compared the symptoms of of patients with "cannabis psychosis", schizophrenia and mania. They reported that no symptoms were unique to "cannabis psychosis" and none that enabled them to distinguish a "cannabis psychosis" from schizophrenia.

Similarly, McGuire collected data which suggests that psychosis which develops or recurs in the context of cannabis use does not have a characteristic psychopathology or mode of onset and is not restricted to a particular ethnic or socio-demographic group. Thus, McGuire concludes "there is little evidence to support the validity of cannabis psychosis as a diagnostic entity".

Beaubruhn & Knight and Stephanis etal conducted two studies which reported no differences whatsoever in the prevalence of psychotic disorder in chronic cannabis users and non-using controls. However, the populations from which the samples were taken (Jamaica and Greece, respectively) have a low prevalence of psychosis anyway, making these findings not entirely convincing.

In a completely different vein, Grinspoon & Bakalar presented the case studies of patients who found cannabis to be effective in the treatment of their bipolar disorder. Some used it to treat the mania, some used it to treat the depression, others used it to treat both. They claimed that cannabis was more effective than conventional drugs in treating their symptoms. An analogy is drawn between the status of cannabis today and that of lithium in the early '50s, when its effect on mania had been discovered but there were no controlled studies. In the case of cannabis, the law has made such studies almost impossible and the only available evidence is anecdotal. The potential for cannabis as a treatment for bipolar disorder unfortunately cannot be explored in the present social climate. Not in the UK anyway!

Most of these studies have concerned themselves with "heavy" cannabis use, so where does that leave the moderate user I hear you ask? Thomas argues, "There has not emerged over the past few decades any convincing evidence that moderate cannabis use leads to any persistent psychiatric disorder, a similar conclusion to that reached by the Indian Hemp Commission more than 100 years ago".

The existence of a "cannabis psychosis" is still a matter for debate. If cannabis induced psychoses do exist, they are rare or they rarely receive medical intervention. While it is a plausible hypothesis that high doses of cannabis may exacerbate or cause a psychotic disorder to occur in those who are already predisposed to psychoticism, a clinical diagnosis of "cannabis psychosis", in terms of a distinct psychotic disorder, is certainly less than plausible.

Louise Schofield 1999