Psychoactive Drugs Tobacco, Alcohol, and Illicit Substances

4. How does drug addiction develop?

• 4.1 How do drug users become drug dependant?
• 4.2 How can genetic factors influence drug use and addiction?

4.1 How do drug users become drug dependant?

The development of drug addiction can be seen as a learning process
Source: WHO

The development of drug addiction can be seen as a learning process. A person takes a drug and experiences the psychoactive effect, which is highly rewarding or reinforcing, and which activates circuits in the brain that will make it more likely that this behaviour will be repeated.

However, the rewarding effects of drugs alone cannot account for why some psychoactive drugs can lead to all of the behaviours associated with addiction.

Similarly, physical dependence on drugs, as evidenced by withdrawal symptoms when drug use is discontinued, may contribute to drug use and addiction, but cannot alone explain why drug addiction develops and is maintained, especially after long periods of abstinence. A complex interplay of psychological, neurobiological and social factors appears to be responsible.

The development of drug addiction can be explained by the effects of the drug on specific areas of the brain that increase an individual’s motivation to use the drug.

The brain has evolved to guide and direct behaviour toward stimuli that are critical to survival such as those associated with food, water, mating, and avoiding danger. These stimuli are recognized by the brain as important because they activate specific “circuits” in the midbrain.

Psychoactive drugs artificially activate these same circuits very strongly and “trick” the brain into responding as if the drugs and their associated stimuli (environments, people, objects) were biologically needed.

With repeated exposure, the brain “learns” the relationship between the drug and drug-related stimuli, causing a stronger and stronger desire or craving for the drug. For example, the mere sight of a cigarette or the smell of tobacco can cause such a strong desire to smoke in tobacco-dependent people that it overwhelms them and makes them relapse to tobacco use, even after long periods of abstinence. More...

4.2 How can genetic factors influence drug use and addiction?

Many environmental and individual factors (including genetic disposition) converge to increase or decrease the odds that a particular individual will use a psychoactive drug, and to what extent.

In addition to social and cultural factors, genetic differences largely contribute to person-to-person differences in psychoactive drug use and dependence. Studies of patterns of genetic inheritance in families, in identical and fraternal twins, and in adopted individuals, provide information on the extent to which inherited factors play a role in drug dependence. Tobacco use, opioid dependence, as well as alcohol use and dependence, have been shown to be significantly heritable. Gene research seeks to identify the genes that are involved.

Dependence on these drugs is caused by the interaction of several genes with environmental factors. Hence, exposure to drugs could have a much greater effect on somebody who carries a genetic vulnerability to drug dependence than on someone who does not. This partly explains why many of those who experiment with drugs at some point in their lives do not become dependent.

The inherited differences involved in drug use and dependence vary for each drug:

• for tobacco, differences in the genes involved in the metabolism of nicotine,
• for alcohol, differences in the genes involved in the metabolism of alcohol and inherited differences in certain brain cell receptors (for serotonin, dopamine and GABA), and
• for opioids, inherited differences in the brain cell receptors for opioids and in the enzymes that metabolize opioids.

Genetic differences may influence many aspects of a person’s drug use. For instance, they may influence how pleasurable a drug is, to what extent it harms health (overdose or long-term effects), how strong the withdrawal symptoms and cravings are, and how the person develops tolerance. More...