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Pleasure and pain
Pleasure-Seeking Behaviour
Pleasure and Drugs
Avoiding Pain

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Research : Jean-Pierre CHANGEUX : Drogues et Communications Neuronales

The potential toxicity of psychoactive substances, like that of any medication, depends on the amount consumed. This toxicity of course varies from one drug to another, but as a general rule, the higher the dosage you consume, the more the substance will have toxic effects on your organism.

An estimated 1.5 million Canadians now smoke marijuana occasionally, for pleasure. This recreational use does not cause any escalation in the great majority of cases.


The human brain contains circuits that provide us with pleasure to reinforce behaviours that are helpful to our survival. The brain thus naturally favours those behaviours that lead us to seek euphoria. One direct way of achieving euphoria is to take psychoactive substances.

The risks associated with consuming a drug of course vary with such factors as the nature of the drug, the vulnerability of the person consuming it, and the amount consumed. But the main determinant of a drug’s long-term impact on the brain is whether it is taken for a short time or a long time. There are three categories of drug use: recreational use, abuse, and dependency. (Roll your cursor over the three traffic lights to read descriptions of each category).

But vulnerability to dependencies is not limited to psychoactive substances. People can also become dependent on compulsive behaviours, such as compulsive gambling or bulimia, which can also lead to an irresistible need to continually repeat the behaviour in question.


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The nicotine in tobacco is a very good example of a substance that causes physical dependency. According to surveys, about half of all smokers would like to quit. About 25% give it a serious try every year. But in the end, only one out of 2 (12% of all smokers) actually manage to do it. In other words, not many, which clearly proves that this is not just a question of will power. While it is true that the nicotine in cigarettes can aid in concentration, act as a tranquilizer, and regulate appetite by binding to sites in many different parts of the brain, it also binds to sites in the reward circuits of the limbic system and thus creates a dependency.

Nervousness and irritability are two sensations commonly experienced when the smoker’s brain no longer receives its accustomed dose of nicotine. For over 30 years, the tobacco companies have vehemently denied any dependency associated with their product. Even today, they use all kinds of deceptions to lure younger people into the vicious circle of dependency. Children must be warned about the risks of tobacco between the ages of eight and ten. By age 12, it is already too late.

The famous methadone programs for heroin addicts let them reduce the suffering of heroin withdrawal, one of the most painful kinds there is. Methadone is a molecule that resembles heroin and binds to the same receptors. But it does not produce the rush of pleasure that heroin does. Similarly, the physical symptoms associated with methadone withdrawal are less severe than those associated with heroin withdrawal. This is why heroin addicts prefer to withdraw from methadone.

Denial and rationalization are two psychological defence mechanisms commonly used by people with drug dependencies.


The behavioural changes that accompany the compulsive consumption of a drug can be better understood if we distinguish between physical and psychological dependency.

Physical dependency is the organism’s physiological reaction to the absence of the drug. It is a state of deprivation that is accompanied by uncomfortable physical symptoms known as withdrawal (see box below).

Physical dependency occurs when the body is deprived of drugs such as opiates (opium derivatives), tobacco, alcohol, and certain psychoactive medications. This deprivation leads to physical symptoms that vary with the drug: pain, in the case of opiates; severe tremors, in the case of alcohol; and convulsions, in the case of barbiturates and benzodiazepines.

In fact, most symptoms of withdrawal from a drug are the opposite of those observed when a person is intoxicated with that same drug. The reason is that the brain has developed compensatory pathways, and when the drug is taken away, they are suddenly overcompensating. These symptoms can also be accompanied by behavioural problems, such as anxiety, anguish, irritability, and agitation.

In the case of certain psychotropics, it can be very dangerous, or even fatal, for someone to suddenly stop taking them if he or she has been consuming them regularly. Thus, people withdrawing from anxiety or sleep medications should do so under the supervision of a qualified person.

Psychological dependency can last far longer than physical dependency: years, or even a lifetime. It is based more on the individual’s traits (habits, affective states, lifestyle) than on the substance itself. It is the memory of the pleasure associated with the object of the dependency that the individual thinks about often and longingly.

Sometimes, the re-emergence of a problem that the person was trying to suppress by taking the drug can cause them to recidivate (start taking it again). People may also remain dependent out of fear of the painful withdrawal that awaits them if they stop taking the drug.

Psychological dependency is often described by the word " craving ", which clearly conveys the intense desire to take the drug and feel its effects. Craving is similar to the physiological sensations of hunger and thirst.

Another psychological factor encouraging drug dependency is the conditioned learning that occurs unconsciously when someone is taking a drug. In this process, both endogenous and environmental stimuli become associated with the desired substance and acquire the power to activate the craving all on their own. For people addicted to injectable drugs, the whole environment surrounding “shooting up” (syringes, “shooting galleries”, etc.) therefore becomes tremendously important.

Simply coming into contact with this environment can cause them to start taking the drug regularly again. So can taking just a small amount of the drug, or being under stress, or even experiencing subjective emotions formerly associated with taking the drug.

Cocaine and amphetamines are good examples of substances that do not cause any physical dependency, but do cause very intense psychological dependency.

Neurobiochemical studies have shown the reward circuit to be an important substrate of psychological dependency. That said, many theoretical attempts to explain the mechanisms of dependency are still being debated today.

Successfully bringing a drug-dependent person through withdrawal is no easy thing. The reason is that repeated consumption of a drug short-circuits normal neurotransmission in the reward circuit. The brains of drug-dependent people adapt to the regular introduction of psychoactive substances from external sources by modifying their production of certain neurotransmitters. The withdrawal syndrome occurs when this external input suddenly ceases. The organism, which has deactivated certain of its metabolic pathways, will then take a certain amount of time before it re-establishes the equilibrium of its neurotransmitters.

There are then two solutions for eliminating the discomfort of withdrawal: take the drug again and allow the dependency to become even more firmly established, or give the organism the time it needs (from a few days to a few weeks) to re-establish the equilibrium of the neurotransmitters in its brain and thus the chance to escape from the vicious circle of dependency.

To continue getting the same sensation of giddiness and euphoria, drug-dependant people often have to keep increasing their dosage. In other words, they need more and more of the drug to get the same effect. This well known phenomenon is known as drug tolerance, or habituation.

Tolerance represents an adaptation at the cellular level. The brain’s neurons will, for example, modify the number or sensitivity of their receptors to adapt to the increased dosage. To experience a pleasurable sensation, the drug-dependent person will therefore have to go beyond this new threshold. This means increasing the dose a bit more, and so on. Tolerance can also be induced elsewhere in the organism. For example, when people drink, their livers activate enzymes that convert alcohol into less harmful substances. This is why they develop a tolerance that forces them to drink considerable amounts of alcohol just to get a little bit drunk.

Though tolerance is regarded as one of the characteristics of drug addiction, it is neither a necessary nor a sufficient condition to trigger a dependency. Some non-psychoactive substances, such as certain hypertension medications, can also induce tolerance. Conversely, there are certain psychoactive drugs, such as amphetamines, for which no tolerance effects are observed.

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