Drug addiction is a complex disease. It is a confluence of many factors that are influencing different aspects of a person's life, leading them towards abuse and addiction despite the personal harm they are doing, and despite the harm they are doing to the people and relationships around them.
While breaking down addiction into two very simple components is potentially misleading, one can at least try to look at addiction from the perspective of both the mind (psychological disease) as well as the body (physiological disease). These are impossibly linked together, and in order to overcome any serious addiction both components must be recognized and treated.
The Psychological Component of Addiction
The psychological component of addiction is of course the mental aspect of being an addict. It is manifested in certain behaviors that the addict performs with increasing frequency. One of the more prominent ones is drug seeking.
Addicts who are 'drug seeking' and their drug of choice is a prescription drug might frequently visit hospital emergency rooms, go doctor-shopping or pharmacy shopping, fail to provide prior medical records for a new doctor who is about to begin treating them, or in some other way go out of their way to deceive medical professionals and convince them to write a specific prescription for them.
The Physiological Component of Addiction
The physiological component of addiction can be defined by dependence and tolerance. Physical dependence can begin in a matter of weeks of chronic use or abuse of a substance. In the case of opioids, the brain is physically altered by addiction by the creation of more and more opioid receptors. The existence of this growing number of receptors leads to tolerance.
Tolerance occurs when a person requires increasingly larger doses of a particular drug in order to produce the same effect. It typically begins with the effect of the dose of the drug in question lasting a progressively shorter period of time and creating a less intense effect. Tolerance—in terms of the rate of its development—will be different in each person.
Therefore the physiological aspect can actually be regarded as a distinct medical problem from either abuse or addiction which can arise merely from using a medication properly and under a doctor's supervision.
In other words, a person can develop a physiological dependence on a certain drug or substance without having abused the substance whatsoever and without being considered an addict.