Most drugs influence the "reward circuit" of the brain, inducing pleasure and flooding it with the chemical messenger dopamine. A well-functioning reward system encourages a person to repeat life-sustaining actions, such as eating and spending time with loved ones. Dopamine surges in the reward circuit encourage enjoyable but harmful behaviours such as drug usage, causing individuals to repeatedly engage in these actions.
As drug usage continues, the brain adjusts by decreasing the capacity of reward circuit cells to respond to it. This diminishes the high a person experiences compared to when they initially began using the substance; this effect is known as tolerance. They may consume more of the medicine to obtain the same effect. These brain changes frequently result in a decline in the individual's ability to gain pleasure from other things, such as food, sex, or social activities.
Long-term usage also alters other chemical processes and circuits in the brain, impacting activities including as learning, judgement, decision-making, stress, memory, and behaviour. Addiction is characterised by the fact that many persons who use drugs continue to do so despite being aware of its negative effects.
