Cannabis and Psychose


A great many studies show that the group of cannabis smokers exhibits an increased prevalence both of psychotic symptoms and of short-lived psychotic illness in the strict sense.

Cannabis smoking can cause psychosis, activate latent psychosis and exacerbate manifest psychotic conditions.

This post discusses the following relationships between cannabis smoking and (near-) psychotic conditions:

  • – Cannabis-caused delirium (acute confusional state)
  • – Cannabis-caused toxic psychosis (cannabis psychosis)
  • – Cannabis-caused functional – non-schizophrenic – psychosis
  • – Can cannabis smoking cause schizophrenia?
  • – Interaction between the effects of cannabis and manifest schizophrenia

The interaction of cannabis with psychotic conditions is doubtless one of the most alarming aspects of what is gathered under the heading “harmful effects of cannabis”. I will here report on the prevailing scientific view of this interaction. Occasionally, I will also take my own clinical experience into account. The principal point to be made, though, is that several studies have found not only a statistical association but also a causal link between cannabis smoking and psychoses or psychotic symptoms.

At the same time, this is an elusive subject. Our knowledge about the nature of psychoses is defective, and the impact on the brain of the substances found in the cannabis plant is a subject we still do not know enough about, despite the great breakthroughs made in recent years. Moreover, there are major difficulties involved in carrying out the long-term studies required. One problematic factor is that until recently, there were discrepancies between countries in the terminology used. Furthermore, it is not always clear from a scientific article what psychotic manifestations are actually referred to in it.

Roughly speaking, what we are discussing here are two fundamentally different psychotic manifestations. Those in the first category are called toxic (they belong to the group of brain-damage syndromes) and involve a situation where cannabis consumption, generally intensive and/or long-term, causes the psychotic symptoms by means of the toxic effect of cannabis. In these cases, the symptoms are provoked in direct connection with cannabis consumption and subside once the supply of the drug is interrupted. Residual symptoms can often easily be made to cease by means of antipsychotic medication, and the patient will recover fully if he or she abstains from further consumption of cannabis or other drugs.

The other category is that of functional psychoses/psychotic manifestations. The word “functional” refers to the absence of organic damage. (Today we would have to say that there probably is an organic component, but more in the form of a subtle vulnerability whose nature we know rather little about [].) Above all, this category covers schizophrenia and schizophrenia-like psychoses, which not rarely run a chronic course.

In the toxic group, direct impact on the brain often manifests itself through the appearance of elements of delirium involving signs such as marked bewilderment and memory disturbance, and this delirium is sometimes so evident that it dominates the picture. In the functional conditions, these symptoms are absent, and – at least in the schizophrenic conditions – there is often a sensation of outside interference with thought. The other psychotic manifestations are often similar.

It should be underlined that what we are dealing with here are the most profound disturbances known to psychiatry; even when they are short-lived, such disturbances can leave marks on those affected and on their families which may remain for many years or even be of life-long duration. By definition, these conditions are of a combinatory nature: there is both an abuse condition and a serious mental disorder. These “dual disorders” are among the most difficult to assess in the whole of psychiatry. Moreover, conditions of this type not rarely make demands on the most costly resources available in the field of psychiatric care.

The following relationships between cannabis and (near-) psychotic conditions are discussed here:

• Cannabis-caused delirium (acute confusional state)

• Cannabis-caused toxic psychosis (cannabis psychosis)

• Cannabis-caused functional – non-schizophrenic – psychosis

• Can cannabis smoking cause schizophrenia?

• Interaction between the effects of cannabis and manifest schizophrenia

Older Scientific and Clinical Reports of an Overall Nature

In addition to the more specific studies that have been carried out into the ability of cannabis to cause different specific psychotic conditions, and the studies of the interaction between cannabis abuse and psychotic conditions, especially schizophrenia, there also exist several reports – many of them from studies carried out in the Nordic countries – which should be seen above all as providing evidence that cannabis consumption can profoundly destabilise neurophysiological processes, thereby causing danger to the individual concerned.

In her dissertation entitled The Prognosis of Drug Abuse in a Sixteen-Year-Old Population, Maj Britt Holmberg found that of those young people who had been consuming large quantities of drugs (almost exclusively cannabis) at the age of sixteen, 10 per cent had a case record as psychosis patients eleven years later. This proportion is of course many times larger than that which would be expected in a normal group of young adults (). In another study, a group of 908 patients were examined from a number of different viewpoints in connection with their admission to two hospitals in London. Of the 496 patients who agreed to undergo an examination including a urine test for cannabis, it was found that among those testing negative for cannabis, 62 per cent were diagnosed as having psychosis, whereas 88 per cent of those testing positive for cannabis received such a diagnosis ().

Bier and Haastrup (1985) found, in their study of all patients admitted during one year to a psychiatric unit at one of Copenhagen’s hospitals, that 30 patients had cannabis-provoked psychoses, i.e. psychoses where it was deemed that cannabis smoking had contributed to precipitating (or causing a relapse into) psychosis. The authors conclude that in a population of 100,000, it can be expected that 15 patients per year will be admitted to hospital with psychoses precipitated or caused by cannabis. The group of patients included both people with no other mental disorder (toxic cannabis psychoses), schizophrenics and people with personality disorders. In the late 1960s and early 1970s, Tennant and Groesbeck (1972) studied American soldiers stationed in Europe. In 1969 a major wave of drug abuse, dominated by strong cannabis preparations (hashish), started among these soldiers. During the years 1969–1971, the authors found that, in addition to a series of other acute negative reactions, the number of cases of “schizophrenic reactions” (acute psychotic reactions which did not necessarily have to lead to what we mean by schizophrenia) increased from 16 in 1968 to 77 in 1971 – i.e. an almost fivefold increase in four years. The researchers’ impression was that the smoking of hashish was a major contributing factor in these psychotic reactions.

A North American study of cannabis-smoking young people in the first half of the 1970s () can be mentioned as an older representative of that minority of studies where no association was found between cannabis smoking and psychosis (or other serious mental disorders). However, owing to the low THC concentrations prevailing at that time, marijuana smokers then ingested, on average, only one-third as much THC as present-day marijuana smokers in the United States.

Recent Studies

Over the past 15 years, the studies showing cannabis to have been the stressor causing or inducing psychotic symptoms or psychotic illness in the proper sense (cannabis psychosis, delirium or schizophrenia) have both increased in number and grown ever more likely to use a refined methodology. All of the studies that I refer to in the following sections on psychosis are examples of this, but I would also like to mention some recent more comprehensive studies. On a general note, it is clear that it is above all because of several on-going prospective and longitudinal studies, which started over two decades ago, that we are able to make analyses and have findings at our disposal today.

Within a major longitudinal research project, a research team from New Zealand are monitoring 1,200 children/young people from birth to – at present – the age of 21 (). It has been found that cannabis-dependent young people develop psychotic symptoms more often than those who are not dependent.

Arseneault et al. (2002) – also a New Zealand research team – participate in another longitudinal project which is monitoring 1,100 children/young people who are now 26 years old. Young adults who smoked cannabis at 15 run a greater risk of developing schizophrenia or schizophrenia-like illness: their risk is 10 per cent, as against 3 per cent for non-smokers. It is also interesting to note that the younger a person is when first using cannabis, the greater is the risk of psychotic manifestations.

It should be added that all studies cited in the following sections on psychosis obviously also support the existence of a causal link between cannabis and psychosis. Two of the most important of these studies are the Swedish one covering 45,000 military conscripts, which is mentioned in the chapter on cannabis and schizophrenia (), and the recent and well-made Dutch study by van Os et al. (2002).

In order to complete the picture and provide a reminder of sorts that this remains a complex field, even though a great deal of clarity has been achieved in recent years, I would like to mention a study by Phillips et al. (2002). The researchers involved carried out a study – albeit of short duration (one year) – on a group of what one would perhaps call pre-psychotic young people, comparing those of them who had smoked cannabis in the year preceding the year of the study with those who had not. No differences were found in the frequency of illness, which is surprising in spite of several objections that the researchers themselves noted could be made against their study, e.g. that the patients were too few, that the doses of cannabis were low, that the patients’ consumption of cannabis during the actual year of the study was unknown and that the study was of short duration.

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