How substances affect the brain

Substances interact with the brain and affect its function in many ways. For example, by changing a person's mental processes and behaviour, substances can affect memory, attention and the way people talk. They can also increase impulsiveness, which can lead to aggression and violence. You may also notice changes in the level of alertness and perception of the world.

Addiction (also called dependence) is a complex illness characterised by compulsive behaviour (the person has a compelling need to use a substance), and uncontrollable cravings (having a strong desire to get the substance). People who are addicted to psychoactive substances persist in using the drug even in the face of extremely negative consequences (e.g. family or job problems, being jailed).

People with addiction typically need increasingly high doses of the drug to achieve the same effect (this is called tolerance), and when they try to stop or reduce the intake of the drug they develop adverse physical and/or psychological symptoms, called withdrawal effects.

Substance abuse is frequent use of a substance despite negative consequences. Not all drug use is bad. Many people have the occasional glass of alcohol and don't develop any negative effects. It is only when someone shows harmful use of substance (e.g. excessive drinking that is damaging to physical and mental health) that problems develop.

Habit-forming substances can be divided into different classes according to their effect (Figure 14.2).

Depressants are substances that have a relaxing effect on people by reducing the activity in the brain. Examples of depressants include alcohol and drugs prescribed for sleep problems.

Stimulants are substances stimulating the brain, e.g. khat and tobacco.

Hallucinogens are substances producing hallucinations, e.g. cannabis may produce hallucinations if taken in large amounts.