Dozens of explanations have been proposed for drug use and abuse. In the early 1980s, the National Institute of Mental Health (NIDA) published a volume that spelled out more than forty theories of drug abuse. The dictionary defines "theory" as an explanation for a general class of phenomena. To most people, a "theory" of drug use, therefore, would be an explanation as to why people use and abuse drugs. However, not all the theories that have been proposed address this issue. Most theories do not attempt to explain the entire spectrum of use; most concentrate either on illicit use (often referred to as "abuse") or on alcoholism. Some focus entirely on addiction, usually to narcotics. Some focus on the drug experience — the consequences of experiencing the drug high in a certain way—and do not deal at all with the question of why people use drugs.

Some focus on the individual; others on society; still others on the individual's relationship to society. While a number of theories deal with initiation into drug use, several focus on continued or habitual use. And nearly all these theories are partial in scope: they select one or a limited number of factors that are believed to cause drug use or abuse. Most theorists admit that the factor they focus on, in combination with others, influences drug-taking. Hardly any researcher in the field believes that one factor, and one alone, explains the phenomenon under investigation.

Moreover, a factor is not a theory; most theories put together several factors to make up a coherent explanation, an argument with several different pieces articulating with one another. All of this means that most theories of drug use are not contradictory or in competition; most, in fact, cover different aspects of the same phenomenon, and may be regarded as complementary rather than contradictory.

There are three broad types of explanations for drug use:

1. biological theories,
2. psychological theories, and
3. sociological theories.

Each focuses on a different range of factors as crucial in determining why people use and abuse psychoactive substances. Of course, within each broad type, there is a range of specific theories.

Before inspecting the theories in different disciplines or fields, it might be useful to mention that the most widely accepted general approach to drug use and abuse—not exactly a theory but a way of looking at the phenomenon—cannot be located in any particular field. It is adhered to by much, probably most, of the public, and by most practitioners, such as physicians, psychiatrists, psychologists, social workers, and therapists, who work directly with drug abusers.

It is called the medical or the pathology model, and its basic assumption is that nonmedical drug use is very much like a disease—a malfunction, an abnormality, a pathology . It is not "normal" to use drugs outside a medical context; only a drug-free existence is normal. No one uses psychoactive drugs to get high unless there's something identifiably wrong with him or her. When things are working right, there's no "need" to take drugs. There is nothing valuable about the illegal drug experience; it is inauthentic, illusory, seductive; nothing but harm can come of it. Drug use should be purged from the face of the earth; it does not deserve to exist.

Not only are drug use and abuse caused by a pathology of some kind, they also cause a wide range of serious pathologies—in other words, "evil causes evil." One cannot fool around with drugs and remain unscathed; there is no such thing as "harmless recreation" when it comes to drugs. Recreational drug use violates the proper rules of nature, science, and medicine. It is the job of the drug researcher to find a way to eliminate it.

This basic assumption, or set of assumptions, underlies not only much popular thinking about drugs, but provides the underpinning for several of the theories to follow. In other words, several of the biological, psychological, and sociological theories discussed below adopt the medical or pathology perspective toward recreational drug use. However, several do not; pathology-normality is a major dimension distinguishing different theories of drug use.