A growing body of research points to the central role of the dopamine system in chemical, and even behavioral, addictions. Drugs such as cocaine, amphetamine, and nicotine have a direct effect on the dopamine system. Other drugs, such as heroin and marijuana may have an indirect effect on this system. The dopamine neurons originate deep in a region of the midbrain called the ventral tegmental area. These dopamine neurons course through the middle of the brain, connecting with a small structure in the forebrain known as the nucleus accumbens.

This system is known as the mesolimbic dopamine tract and is a central part of the dopamine reward system, which seems to be involved in the activation of the organism to pursue rewarding stimuli. In other words, this system is central to an animal’s experience of desire and motivation. Activation of the reward system stimulates pleasurable feelings of euphoria, energy, and enthusiasm. Many drugs of abuse directly stimulate this chemical system, providing an immediate and intensely pleasurable experience. In effect, they mimic the brain’s natural chemicals.

Unfortunately, nature allows no free lunch. Over time, activating the dopamine reward system by outside chemicals changes the structure of the brain, reducing its ability to regulate the dopamine system.