Cannabis Psychosis


Cannabis smoking, especially of preparations with a high concentration of THC, can cause a toxic and short-lived (between a few days and six weeks) psychosis. Not infrequently, this psychosis has dramatic symptoms and requires hospitalisation, sometimes under constraint.

If the individual concerned stops smoking, the condition has a good prognosis. If he or she continues to use cannabis, however, the psychotic condition can probably remain for a very long time.

Different assessments and calculations to determine the frequency of this side-effect have yielded surprisingly similar results. As a “rule of thumb” it is considered that, on average, a person who smokes cannabis runs a 10 per cent risk of being affected at one time or another by cannabis psychosis or delirium.

Toxic cannabis-caused short-lived psychosis can sometimes be difficult to distinguish from the toxic cannabis-caused delirium described above; moreover, in the initial stage, the two conditions sometimes merge into each other. The essential difference between the delirium and the psychosis is the clouding of consciousness that accompanies the del irium; in the psychotic condition, the individual’s consciousness is by and large clear. Since the course of the psychosis generally lasts longer (one to six weeks) than the intoxication state, one may expect to find a delayed toxic effect. The duration indicated above is valid only if the individual stops smoking. If he or she persists in using cannabis, this psychotic condition, which otherwise responds very well to treatment, may continue for a very long period, perhaps for several years.

The clinical picture can sometimes be very difficult to distinguish from that of acute schizophrenia. However, some researchers maintain that the typical picture of cannabis psychosis does differ from that of schizophrenia and is characterised above all by what are known as “positive” symptoms such as paranoia and other delusions, hallucinations and, not rarely, sudden affective changes where aggressiveness alternates with euphoria. The clearest differentiating factor distinguishing cannabis psychosis from acute schizophrenia is the absence of disruption to thought, which is more or less the rule in schizophrenic patients (). A further difference, which is also the primary one, lies in the subsequent course taken by the condition: cannabis psychosis is short-lived, while by definition the course of schizophrenia lasts for at least six months (according to the Diagnostic and Statistical Manual of Mental Disorders [DSM-IV]). Schizophrenia is often associated with a “premorbid personality”, which can manifest itself in various forms; the most common ones include extreme reserve, loss of earlier interests, vague delusions such as suspiciousness, and bizarre ideas. However, some of these symptoms may also characterise a chronic hashish smoker, which makes differential diagnosis more difficult. The absence of hereditary factors (for schizophrenia) is a further pointer towards identification as cannabis psychosis.

The most certain way of distinguishing between the two kinds of psychosis is to monitor the course taken by the psychosis. As mentioned above, provided that the person stops taking cannabis (as well as any other hallucinogenic or CNS stimulant drugs), cannabis psychosis is short-lived. However, this also means that, if the person continues to smoke hashish or marijuana, thus ensuring the persistence of the psychosis, there is sometimes no certain way of distinguishing cannabis psychosis from schizophrenia or other forms of functional psychosis.

This condition, like the delirium described above, is very frightening both to the individual concerned and to his or her family. Even though this is, from a psychiatric point of view, a condition whose prognosis is good and whose course is generally short-lived, the onset of cannabis psychosis must still be considered a very serious condition. It often requires hospital treatment, not rarely involving hospitalisation under restraint, and during the intensive phase of the psychosis there is probably an increased risk of suicide, for instance by jumping from a height (see the chapter on depression and suicide). Even where the course taken by the psychosis is not dramatic, the subjective impression of the person involved is that the condition represents a temporary mental breakdown, and this is an experience which can cast a shadow over a large part of that person’s future life.

Scientific and Clinical Reports

The scientific literature has featured controversies about whether or not cannabis psychosis exists. In several cases, there has in fact been a mix-up with the delirium described above – a mix-up which is understandable since the two conditions, especially in the initial stages, tend to follow similar courses (). Most researchers, however, are of the opinion that the phenomenon as such – a toxically provoked psychotic reaction – does exist, and “cannabis-induced psychotic disorder” is indeed included as a diagnostic unit in DSM-IV.

And to complicate matters still further: in several research reports, especially older ones, “cannabis psychosis” refers to a longer-lived, functional and non-toxic (except possibly in the initial stages) condition. This means that in many cases, the lack of agreement has related to a condition other than the one I have described here. We will return to these issues in the following sections.

In my opinion, “cannabis psychosis” is an excellent, clinically relevant and accurate term for a short-lived, mainly toxic, cannabis-caused psychosis.

There is a large body of reports from researchers who have themselves studied the issue and from clinicians who have collected descriptions of groups of cannabis psychoses (as defined above). A few older examples of such descriptions are Weil, 1970; Bernardsson & Gunne, 1972; Pålsson, Thulin & Tunving, 1982; Tunving, 1985; Carney, 1984; and Brook, 1984. As the existence of this type of psychosis has been gaining acceptance, at least in the Nordic countries (), such reports have become rarer, while the question of whether cannabis can cause chronic psychosis has come more to the fore. In the United Kingdom, however, where there has been more debate on whether cannabis is dangerous at all, a number of reports on cannabis psychosis have been published (); both of these reports mention very high proportions of habitual smokers having developed delirious and/or psychotic reactions.

With regard to methodologically more advanced scientific studies showing that these psychoses exist and making comparisons with control groups of patients not affected by cannabis, it is above all the studies by Rottanburg et al. (1982) and Rolfe et al. (1993) that deserve mention. Chopra et al. (1974) have also, in a large-scale study, contributed extensive knowledge of cannabis psychoses. On the basis of in-depth analyses of published research findings, Ghodse (1986) and Thornicroft (1990) conclude that cannabis, especially in high doses, can provoke toxic psychoses.

An example from the author’s own clinical work:

While I was collecting material for the first version of the present report, I was also professionally active in a private general psychiatric practice. A young patient (23 years old) was referred to me. He displayed the full range of signs described above: no history of mental illness, no divergent personality traits prior to this disturbance and no schizophrenia in close relatives. This patient’s illness had developed after a period of intensive cannabis smoking. He suffered from pronounced ideas of persecution, “magical” imaginings, pronounced suspiciousness and impulsive outbreaks involving aggressiveness towards members of his family as well as severe sleeplessness (being awake all through the night). By organising round-the-clock care and supervision within his family, making sure he saw me frequently and could phone me at more or less any time of the day or night, and quickly starting treatment with antipsychotic medication, it was possible to avoid having him admitted to a hospital. The psychotic symptoms proper faded after 7–10 days, after which there followed a convalescence period lasting for a couple of months, as well as a few relapses.

Some Recent Studies

As has been mentioned before, increasingly strong support has been found for an association between cannabis smoking and psychoses. Often, however, no clear distinction is made between different psychotic manifestations. In the above-mentioned studies (), it can be assumed that some of the cases did indeed involve cannabis psychosis. As regards the Arseneault et al. study, though, this applies only in so far as the 26-year-olds studied were still cannabis consumers; the report is unclear on this point.

Two teams of researchers () have been able to confirm previous findings where a distinction, based on several features, had been drawn between two different groups of psychoses: acute schizophrenia on the one hand and short-lived toxic psychosis – cannabis psychosis – on the other.

How Common is Cannabis Psychosis?

How common, then, is cannabis psychosis? We do not know for certain. There is probably some degree of underdiagnosis. Most studies from across the world show that there is a tendency at general psychiatric clinics to underestimate the prevalence of drug abuse (including alcohol abuse) in patients. It is a well-known fact at locked psychiatric wards and prisons that it is well-nigh impossible to keep the institution free from drugs; exceptions to this rule are specialised clinics and some special prison wards.

It is probably not unusual for patients to continue taking cannabis (unbeknownst to the staff) even when they have been admitted to a hospital. This will give rise to a longer-lived, schizophrenia-like course of illness. Another reason why the recorded diagnosis may not be cannabis psychosis is that poly-drug abuse is extremely common. If the individual concerned has been using, for instance, both cannabis and CNS stimulants – a frequent combination in Sweden –, we can no longer talk about a pure cannabis psychosis, and in such cases the diagnosis is usually determined by the “harder” abuse. Sometimes the condition can be dealt with in outpatient psychiatric care, and then – in Sweden at least – no note at all is made of the diagnosis in any central register.

For the above reasons, then, it is a difficult task to estimate the prevalence of this condition. Notwithstanding the difficulties involved in distinguishing cannabis psychosis from two neighbouring conditions – on the one hand, “psychotic symptoms” (serious enough, but not sufficiently so to warrant classification as a true illness), and on the other, acute schizophrenia –, several attempts have been made both to estimate and to measure the prevalence of cannabis psychosis.

Based on long experience and a literature review, Johnson (1991) estimated that 10 per cent of all those who had used cannabis on more than a single occasion experienced cannabis psychosis or delirium. This proportion seemed high at the time, but later assessments have confirmed rather than refuted it. The most interesting study may be that carried out by Thomas (1996), who sent questionnaires to 1,000 New Zealanders aged 18–35 years, receiving a reply from 65 per cent. Of these, just under 40 per cent had used cannabis; and of these, 15 per cent had experienced psychotic symptoms. The now commonly accepted rule of thumb is that 10 per cent of those who smoke cannabis risk being affected by psychotic symptoms, which in many cases will amount to cannabis psychosis proper. Johns (2001) also supports this assessment.

In a British report () describing observations of a wave of psychosis and confusional states, it is emphasised that these observations were made in a group of abusers consuming (Dutch) cannabis with a very high THC content. In other words, the risk which is quantified above becomes greater as the intensity of smoking increases, i.e. if the individual concerned smokes more often or uses preparations with a higher THC concentration.

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