The Addict

The following source has a definition for the term addict that I found to be quite fitting:

"Addict." Gale Encyclopedia of American Law. Ed. Donna Batten. 3rd ed. Vol. 1. Detroit: Gale, 2010. 94. Opposing Viewpoints in Context. Web. 26 Mar. 2014.
"Any individual who habitually uses any narcotic drug so as to endanger the public morals, health, safety, or welfare, or who is so drawn to the use of such narcotic drugs as to have lost the power of self-control with reference to his or her drug use."

The following source outlines how addiction occurs biologically:
"Introduction to Addiction: Opposing Viewpoints." Addiction. Ed. Louise I. Gerdes. San Diego: Greenhaven Press, 2004. Opposing Viewpoints. Opposing Viewpoints in Context. Web. 26 Mar. 2014.
"Craving, increased tolerance, and withdrawal were considered clinical evidence of this dependence. Craving involves an intense desire for the drug, tolerance means that the user needs more and more to achieve the high, and withdrawal is the physical and mental suffering that occurs when drug use is discontinued."
"Drug research revealed that drugs mimic or block the brains neurotransmitters in the reward center and take over the brain's dispensing of the reward—the good feeling people get when they eat or have sex. Psychology professor Alice M. Young explains: 'Drugs such as alcohol, nicotine, heroin and cocaine may short-circuit the natural reward pathways that have evolved to ensure that we engage in activities critical to our survival.' Thus, in order to get the "reward," the good feeling people often attribute to being high, people must continue to use the drug."