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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

As earlier noted, the probability of the occurrence of these categories of
harmful effects also depends on how much of the substance is used, in what
forms, and with what patterns of use. These aspects of use may be thought of
as linked to the different kinds of health and social problems by three main
mechanisms of action (see Fig. 1.2). One mechanism concerns the direct toxic
effects of the substance, either immediate (e.g. poisoning) or cumulative over
time (e.g. cirrhosis). A second mechanism concerns the intoxicating or other
psychoactive effects of the substance. A traffic accident may result, for
instance, from the fact that a car driver is under the influence of sedatives. A
retail store employee may be intoxicated at work after using cannabis, and
because of this, may be fired by the manager.
The third mechanism concerns dependence on the substance. Substance
dependence – or dependence syndrome – is the current technical terminology
for the concept of “addiction”. At the heart of this concept is the idea that the
user’s control over and volition about use of the drug has been lost or
impaired. The user is no longer choosing to use simply because of the
apparent benefits; the use has become habitual, and cravings to reuse mean
that the user feels that the habit is no longer under control. The user’s
dependence is thus seen as propelling further use despite adverse
consequences which might have deterred others who are not dependent, from
further use.
The link between substance use and harm in a particular case may, of
course, involve more than one of the three mechanisms. Benzodiazepines
may be involved in a case of suicide, for instance, both through the user’s
despair over the disruption brought to his or her life by dependence on the
drugs, and as the actual means of suicide through overdose. However, the
mechanisms can also operate alone. It is important to keep in mind, moreover,
that dependence is not the only mechanism potentially linking substance
use to health and social harm.

Substance dependence in relation to neuroscience
Social historians have found that the concept of dependence has a specific
history, becoming a common idea first in industrialized cultures in the early
nineteenth century. The term was initially applied to alcohol and later
extended to apply to opioids and other psychoactive substances (Ferentzy,
2001; Room, 2001). In the case of alcohol, the equivalent term became
“alcoholism” by the 1950s, while general application of the concept of
dependence on tobacco smoking is more recent. While the general idea of
dependence is now well established in most of the world, comparative
research has found that there is substantial variation between cultures in the
applicability and recognition of specific notions and concepts associated with
it (Room et al., 1996).
As defined in The ICD-10 classification of mental and behavioural disorders,
substance use dependence includes six criteria (see Box 1.2); a case which is

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