Damage to Cognitive Mental Functions While under Chronic Cannabis Influence


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

Scientific Studies

In 1986, Wert and Raulin made two wide-ranging reviews of all studies that had been carried out in this field up to that point. They found () that neither neurological nor neuropsychological studies had shown unambiguously that chronic abusers suffered from structural or functional damage caused by their chronic abuse.

However, Wert and Raulin do discuss the possibility that the “differential impairment” (in groups equally exposed to cannabis abuse and having experienced similar conditions in other respects, some subjects exhibited damage while others did not) found in many studies might not be a consequence of faulty study design but rather a manifestation of varying vulnerability in different individuals. The authors write:

It might well be that some individuals are predisposed to cerebral impairment as the result of cannabis use, either because of structural or biochemical characteristics which accentuate the possible damaging effects of the drug, or because they have little of the cerebral reserve that most of us call on when we experience mild cerebral damage. That functional reserve can mask very real cerebral damage.

This kind of interaction between stress/damage on the one hand and vulnerability on the other is now an accepted model for explaining both how many illnesses arise and why some people do not fall ill. Generally, however, we know very little about these vulnerability factors in each individual case. This line of thinking is obviously valid for many of the harmful effects dealt with in this report.

With regard to what were, after all, the dominant findings – no proven damage as a result of prolonged cannabis use –, it could not be excluded that the testing methods may not have been sensitive enough. However, Wert and Raulin’s answer to that objection was that it was indeed possible, using the same testing methods, to detect brain damage in alcoholics.

Subsequent studies have proved them right: the damage caused by cannabis smoking is not only of a more subtle kind than had previously been expected, but it is also different in nature.

A couple of studies returned to earlier study groups where the original study had not been able to prove the existence of any cognitive damage. Renewed testing which made use of more sophisticated methods found clear differences between abusers and non-users, especially with regard to the ability to sustain attention and the ability to remember something just learned (short-term memory) ().

By using more specific testing methods and applying a more rigorous methodology, a number of studies have shown that prolonged use of cannabis causes damage in the cognitive sphere, particularly with regard to specific aspects of memory and attention as well as the organisation and integration of complex information. In order to exclude the effects of acute intoxication, the tests were carried out after at least 24 hours’ abstinence from cannabis smoking. In general, there was found to be an association between the duration of the cannabis-smoking habit and the degree of functional impairment measured: the longer the period of abuse, the worse the test score.

Block et al. () showed that intensive, prolonged cannabis smoking is detrimental above all to the ability to express oneself verbally and to solve mathematical problems. Solowij () has shown that prolonged cannabis use leads to impaired ability to focus attention and to screen out irrelevant information. Schwartz et al. () showed, in a study which is discussed in greater detail in the chapter on teenage development (), that cannabis smoking resulted in a significant impairment of short-term memory which persisted for at least six weeks after the individual stopped smoking. Leavitt et al. have presented their findings only at scientific conferences (reports which I have not had the opportunity to read), but their results are reported in reasonable detail by Hall et al. () as well as by Lundqvist (), who concurs in the opinion of Leavitt et al. that they and other researchers have shown that long-term use of cannabis entails, among other effects, the following:

  • impaired ability to carry out complex thought operations and impaired ability to screen out distracting impressions;
  • reduced ability to process information;
  • no effect on long-term memory but impaired short-term memory, particularly with regard to information which is of a kind unfamiliar to the individual or which is complex in nature;
  • difficulty in carrying out tasks which require intellectual flexibility, long-term strategic planning and the ability to learn from experience;
  • no effect on the ability to deal with the routine, familiar demands of everyday life, but problems when faced with the task of expressing oneself verbally in a new, unfamiliar situation or in a situation where old ways of thinking and old knowledge are inadequate.

With regard to the question of whether functional impairments can persist over a lengthy period of time after discontinuation of abuse, Solowij () has shown that a “reduced ability to focus attention and to screen out irrelevant information” remains after the individual has stopped smoking cannabis. When Solowij’s study was carried out, the subjects had refrained from taking drugs for an average period of two years. There was found to be a direct relationship between the degree of impairment and the duration of cannabis abuse.

Clinical Observations

In his PhD thesis, Thomas Lundqvist () looked at the cognitive damage arising in connection with prolonged cannabis smoking. He presented a model for categorising the cognitive functions at issue, a model which he had been using for a number of years to organise clinical observations of cognitive functions in 400 long-term cannabis abusers who had sought care at an outpatient clinic. Together, these clinical observations provide a very informative illustration of and supplement to the scientific studies referred to above. In a small-scale study (which will be accounted for in the next section), Lundqvist sheds more light on the disturbances to chronic abusers’ experience of the surrounding world.

Lundqvist’s clinical observations regarding cognitive disturbances, when placed in relation to scientific studies, lead both to a treatment model for cannabis abusers and to a number of interesting hypotheses and figures of thought concerning the direct effects of cannabis on the various structures and functions of the brain. However, those parts of the thesis fall outside the scope of this report.

It is worth noting in its own right that over 1,000 cannabis abusers have sought help at the treatment centre where Lundqvist works. Even though the functions necessary to assess their need to stop abusing will have deteriorated as a result of their abuse, many cannabis abusers still wish to make an attempt to change their life situation. This is a reflection of the fact that cannabis abusers suffer from their dependence and their functional impairments.

Lundqvist divides the cognitive functions which are impaired when the individual is under the influence of cannabis into the following categories:

Verbal ability

Having a vocabulary that corresponds to one’s age, finding the words for what one wants to say, understanding others and having the ability for abstract thought.

Logical-analytical ability

Ability to analyse and draw logical conclusions, ability to understand causal connections and ability to judge oneself in a critical/logical manner.


Ability to maintain attention and to vary the degree and focus of attention. Ability to understand other points of view and to change one’s own point of view. Some degree of general flexibility with regard to different ways of looking at and interpreting societal phenomena.


  • Short-term memory/working memory: Ability to remember what has just happened or been communicated, which is a prerequisite not only for the integration of what has just been communicated but also for the integration and organisation of a whole range of cognitive processes, as well as a precondition for a reasonably adequate temporal perception.
  • Long-term memory: This consists of both “episodic memory”, which makes it possible to remember events and their temporal context, and “semantic memory”, which has more to do with what we call “knowledge”, e.g. different facts and the inter-relationships between different phenomena.

Analytical and synthetic ability

Based on the ability to combine the other functions. Makes it possible to synthesise, sort out and organise mental material.

Psychospatial ability

Makes it possible to orient oneself, other people and various phenomena in time and space, which is a precondition for temporal organisation as well as one of the prerequisites for social orientation.

Gestalt memory (holistic memory)

Enables us to understand and form patterns – not only to understand that there is a connection, but also to understand its nature and structure. For example, enables us to make and maintain the connection between a person, a name and a social role.

All of these functions were disturbed, to a greater or lesser degree, in the cannabis abusers who sought help at the treatment centre. Systematic interviews were carried out with ten former chronic cannabis abusers, between 2 and 48 months after they had discontinued their abuse, about the changes they felt they had gone through. All of them said that their way of thinking and the way they perceived the world around them had changed after they had stopped smoking. Above all, they felt that their verbal ability, their logical-analytical ability and their psychomotility had improved.


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