Psychoactive (or psychotropic) drugs affect the central nervous system. They can alter the function of the brain, changing one's behavior, mood, thoughts, perception, and consciousness.
Substance dependence (or addiction) occurs when a user is dependent on a drug to function and has a physical or mental addiction to the substance. Neuroscience research has determined that this dependence has genetic and biological origins, making it a disorder and not simply a choice.
It is widely held by the International Classification of Diseases system and the American Psychiatric Association that there are various criteria to determine addiction to a substance. Among those are tolerance (requiring more and more of a substance in order to reach the same effect), withdrawal (physical and psychological effects of reducing or complety stopping the intake of a substance), and the social effects of continued use of a substance.
Because of the effects of psychoactive drugs on the central nervous system, tolerance, withdrawal, and social effects are highly affected by the use and disuse of these drugs. Over the years, these effects have appeared more frequently and with a higher intensity. This is due in part to a manipulation of drugs, many of which were designed to either get around regulations or to appear similar to regularly available prescription drugs. During this process, drugs were sometimes packaged together along with other compounds with less of a high but more of an addictive quality. For manufacturers and dealers, the use of more highly addictive drugs ensured repeat customers.
In juxtaposition, it is less likely for a user to become addicted to a non-psychoactive drug, even when the effects are pleasant. For example, let's take acetaminophen. This drug is used to treat pain. When there is no pain present, there is no effect of the drug. Since there is no effect on the central nervous system, this drug is less addictive. It does, however, lead to tolerance when used on a regular basis. From this effect we can see that acetaminophen does fall into some, but not all, of the qualifications to make an addictive substance.