Dopamine, serotonin, GABA and opiate peptides are necessary to experience feelings of euphoria, energy, self-esteem and a sense of well being. Substances of abuse, such as heroin, cocaine and alcohol, cause a magnification of the effects of these neurotransmitters by over stimulating the receptor sites in the brain associated with each neurotransmitter. The elevation in mood caused by this overstimulation is so pleasurable that it creates a desire for continued use of the substance. However, when overstimulation occurs, the brain reacts by reducing production of the neurotransmitters to attempt to maintain a normal level of neurotransmitter activity.

​

Consequently, more frequent and higher doses of the substance of abuse are necessary to induce pleasure. Furthermore, when the pleasurable "high" from the substances wears off, the brain experiences more acutely the lack of neurotransmitters caused by reduced production, and symptoms such as depression and anxiety begin. At this point, memories of the euphoric experience of the abused substance cause a craving for more of the substance, and the cycle of use continues.