Harmful Effects of Cannabis Smoking on Reasoning Ability, Memory and Sense of Coherence (Cognitive Functions)


After a single instance of intoxication, the acute psychotoxic effects caused by cannabis smoking on cognitive functions (reasoning ability, memory functions, analysis and planning ability, etc.) will remain, in general, for a maximum of four to five hours. The duration of these effects is dependent on the level of THC in the blood, and there is a delay of an hour or so relative to the time of consumption. In the case of repeated consumption, i.e. on one or more occasions per day, the functional impairments will persist (even though the individual learns how to hide certain functional shortcomings) and the entire personality will eventually become marked by above all cognitive difficulties, and also by the social strategies to which the individual has recourse in order to cope. The manifestations of the chronic effects include the following: decreased ability to carry out complex thought operations, reduced ability to concentrate, decreased ability to process information, impairment of short-term memory, reduced intellectual flexibility and ability to learn from experience, lowered ability to carry out long-term strategic planning, and difficulties expressing oneself verbally in new, unfamiliar situations where old modes of thought and old knowledge are inadequate.

Chronic abuse also leads to a measurable deterioration in the individuals’ “sense of coherence” (i.e. their perception and understanding of their environment), although this effect fades a number of weeks after abuse is discontinued – or sooner if treatment is given.

Systematic clinical observations of cannabis abusers yield a picture of a personality characterised by rigidity, inflexibility and memory problems – and consequently problems in dealing with change. Cannabis abusers are not seldom lonely and isolated people.

Studies of chronic abusers show that certain functional defects (typically subtle ones) persist for several years after the individual has discontinued his or her abuse. The functions affected relate mainly to memory and attention.


The effects of cannabis intoxication include diminished memory, changes in temporal perception and reduced ability to pay attention. This affects a number of sensorimotor functions such as reaction speed, co-ordination, the ability to assess lengths of time and the ability to carry out several tasks simultaneously. More complex functional impairments can also arise (probably through interaction between the above-mentioned defects), especially in the case of long-term – i.e. more than occasional – abuse.

The acute deterioration of these functions and its importance for (above all) driving will be discussed in site. The cognitive disturbances might possibly be at the core of the mental weaknesses which provoke psychosis and depressive symptoms. Post accounts for late mental effects on children whose mothers have smoked cannabis during pregnancy. The symptoms exhibited by these children are similar in many respects to some of the cognitive disturbances which arise in cannabis-smoking young people and adults.

This site looks at the question of how chronic cannabis abuse affects mental functions – i.e. what happens to human intellectual and psychomotor functions if the brain is exposed to constant and very prolonged cannabis intoxication. It is reasonable to assume that repeated poisoning, month after month and year after year, will produce some form of negative effects, and it is also reasonable to assume that those effects do not consist solely of the repetition of the effect of acute poisoning.

A particularly interesting aspect of this relates to the effects it may have on personality: on the experience of self, on the individuals’ perception of their environment, on their ability to function psychosocially at the psychological and social levels, on their ability to develop personal maturity, and so on.

Studies and observations can be classified as laid out below, according to the types of harmful effects to which they refer:

  1. a) Permanent brain damage following cannabis smoking
  2. b) Damage to cognitive mental functions while under chronic cannabis influence
  3. c) Effects on complex mental functions such as the sense of coherence and the ability to process new impressions and one’s own memories

a) Permanent Brain Damage following Cannabis Smoking

The term “brain damage” as used here refers to permanent damage which can be demonstrated radiologically while the individual is alive or by means of microscopical investigation of the brain of a deceased abuser.

There are many clinical and anecdotal reports describing prolonged impairment of mental functions in chronic abusers of cannabis, not least from developing countries. These observations are supported by early scientific studies (). Even though these studies have severe limitations due to unsatisfactory scientific design, they contributed strongly to placing the discussion of permanent brain damage on the scientific agenda at an early stage.

Campbell et al. () caused something of a sensation by publishing a study where they showed the existence of cerebral atrophy (using air encephalography) in ten chronic cannabis abusers while thirteen controls of the same age returned normal results. However, in addition to the fact that this study was severely criticised on several counts from a methodological point of view and proved to be unrepeatable by other researchers, it has been shown in several studies using modern radiological methods (computer tomography) that cerebral atrophy cannot be detected in chronic cannabis abusers ().

b) Damage to Cognitive Mental Functions While under Chronic Cannabis Influence

c) The Effect of Cannabis Abuse on the Abusers Understanding of the Surrounding World as Expressed in Their Sense of Coherence

Antonovsky () has developed “sense of coherence” (SOC) into a clinical and scientific notion. He found that people who had been better than others at dealing with demanding, traumatic experiences in their lives had a more developed sense of coherence. SOC can be seen as constituted of three components: comprehensibility – being able to understand the situation in which one finds oneself and the traumas one is exposed to; manageability – being able to handle a situation or act upon it, or being certain that somebody whom one trusts will act in one’s interests; and meaningfulness – in the sense that the strains to which one is subjected are in some way meaningful.

Antonovsky has developed a test which measures a person’s degree of SOC. Lundqvist tested fifteen cannabis abusers on two occasions – when they were admitted and six weeks later, i.e. after six weeks of both abstinence and treatment. On admission, the average test score was 118.2; six weeks later it had risen to 141.9. This represents a statistically significant improvement, both for SOC as a whole and for each of its three components. Following treatment, the former abusers obtained scores close to those achieved by the control group consisting of non-abusers with university degrees, whose average score was 153. (According to Antonovsky, in such a country as Sweden, scores between 143 and 153 are to be seen as reflecting a good level of adaptation in these respects.) These improved levels were recorded although it can reasonably be assumed that the recently detoxified former abusers were not yet in a state of full mental balance – a circumstance which has been shown by experience to result in lower SOC scores. It was also found that cannabis abusers who had undergone a period of abstinence but had not received any treatment obtained a significantly worse score than the study group.

Even though this is a single study – and taking into account that it is not entirely clear exactly what is measured by the test –, this research into the understanding of the surrounding world supports the position that prolonged cannabis abuse weakens the individual’s ability to maintain a functional relationship with the world around him or her.

What Is the Importance of the Cognitive Damage to the Individual?

Even though the mechanisms involved are unknown to us, it seems reasonable to assume that the impairments in cognitive functionality play a significant part in several of the psychiatric harmful effects described in the opening chapters of this report.

In post, “Cannabis Smoking in Teenagers”, I discuss the impact on the individual of prolonged cannabis smoking during the teenage years, especially as regards its effects on learning/schooling and on psychosocial development.

The impact of this cognitive damage on the individual’s ability to operate complex machinery in the processing industry and, above all, to operate vehicles – cars and aeroplanes – in traffic is the subject of post, “Cannabis and Driving”.

Obviously, a reduction in memory capacity affects the learning ability, in a broad sense, of adults as well as that of children and young people. There is good reason to recall Wert and Raulin’s thoughts with regard to “differential impairment”, which we looked at in the previous section. The nature and severity of the harmful effects produced are doubtless dependent in large part on the extent to which the individual is able to compensate mentally and socially.

Lundqvist () found, in his clinical assessment of the 400 chronic cannabis abusers, that most of them displayed more or less pronounced weaknesses in all seven cognitive categories (and certain emotional disturbances also seem to be part of the picture).

Moreover, Lundqvist claimed that he could distinguish a typical personality profile characteristic of cannabis-smoking clients. According to this profile, abusers typically:

  • have difficulty in finding the words to express what they really mean;
  • have a limited ability to be amused by or enjoy literature, film, theatre and the like;
  • have a feeling of boredom and emptiness in everyday life, along with feelings of loneliness and of not being understood;
  • externalise problems and are unable to take criticism;
  • are convinced that they are functioning adequately;
  • are unable to examine their own behaviour self-critically;
  • feel that they have low capacity and are unsuccessful;
  • are unable to carry on a dialogue;
  • experience difficulty in concentrating and paying attention;
  • have rigid (fixed) opinions and answers to questions;
  • make statements such as “I’m different, other people don’t understand me, I don’t belong to society”;
  • do not plan their day;
  • think they are active because they have many on-going projects – which they seldom see through to completion;
  • have no daily or weekly routines.

Lundqvist bases what he has to say on his extensive clinical experience of how a new identity develops in the chronic abuser. Even if one may not wish to be quite so categorical, what emerges is clearly a picture of rather special personality traits: rigidity, inflexibility, difficulty in remembering and thus generally in dealing with changes in the surrounding world, loneliness and a tendency towards isolation. These personality traits are doubtless dependent both on the size of the dose and on the duration of abuse; but the cognitive, emotional and social resources originally available to the individual also play an important part. The similarities with “amotivational syndrome” () are striking.

Having previously studied young people, Hendin et al. () wished to determine the consequences of continued cannabis use in a group of adult habitual smokers. Subjects in a carefully selected group of 150 long-term users of cannabis were asked about their subjective impression of various effects caused by their long-term smoking. These subjects were white, did not abuse any other drugs or alcohol, and were not marginalised or socially disadvantaged. They had been using cannabis at least six days a week for at least two years. The aim of the study was to increase understanding of the role played by cannabis in these people’s lives, and special interest was directed towards adaptive aspects, which were later studied in greater detail in 15 specially selected subjects.

Alongside a number of aspects subjectively felt to be positive (of which, however, some were found, in the subsequent intensive study, to be objectively negative), two-thirds of the 150 subjects felt that the main disadvantage of chronic abuse was memory impairment. Just under half were of the opinion that their ability to concentrate on a complex task had deteriorated, and an equal proportion considered that an inability to get things done was one of the negative long-term effects. Further, 43 per cent felt that there had been a deterioration in their ability to think clearly, and 36 per cent considered that their level of ambition had become lower as a result of their chronic abuse. Moreover, some of the subjects interviewed felt that acute intoxication had an additional negative effect on functions such as memory (45 per cent) and ability to concentrate (41 per cent).

The intensive study of 15 specially selected chronic abusers, which Hendin et al. carried out within a psychoanalytic frame of reference, is of considerable interest; for the main part, though, it falls outside the scope of this report. One of the findings, however, will be mentioned here. Cannabis smoking is often claimed (by abusers themselves) to have the effect of increasing one’s self-awareness and of stimulating one to contemplate and gain a deeper understanding of one’s own and other people’s situation in life. The researchers were in fact struck by how consistently chronic marijuana smoking was found to have the opposite effect! Introspection was effectively inhibited, thought and feeling were separated, and the individual became less able to see reality. Cannabis was used, then, as a means of escaping from an awareness which might have provided the individual with a basis for maturing, for making conscious choices in life and for dealing with disturbed relationships.


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