Dopamine system - drugs act to change the way neurotransmitters operate the brain. Most psychoactive drugs of addiction work on the dopamine system. For example, heroin increases the amount of dopamine in the reward pathways of the brain by boosting the activation of the dopaminergic synapses causing an intensely pleasurable experience or feeling of euphoria. However, the brain naturally reacts to the sudden increase of dopamine but reduces its natural dopamine production, but when drugs' effects on the dopamine system wear off, people have less dopamine than normal brain function. Causing the unpleasurable experience (dysphoria), people are motivated to take more heroin, so they stop feeling bad, this then reproduces the high they first felt, making them physically dependent in order to avoid the negative experience of withdrawal, which is caused by the lack of dopamine, now produced by the brain and so leads to addiction.
