scienceisbohring:

One of the neurotransmitters playing a major role in addiction is dopamine. Dopamine affects brain processes that control movement, emotional response, and the ability to experience pleasure and pain. Regulation of dopamine plays a crucial role in our mental and physical health. Cocaine and other drugs of abuse can alter dopamine function. Such drugs may have different action depending on which dopamine receptors the drugs stimulate or block, and how well they mimic dopamine. 

(Source: wikipedia.com, via fuckyeahneuroscience)