Substance abuse can simply be defined as a pattern of harmful use of any substance for mood-altering purposes. Medline's medical encyclopedia defines drug abuse as "the use of illicit drugs or the abuse of prescription or over-the-counter drugs for purposes other than those for which they are indicated or in a manner or in quantities other than directed."

But the broad range of substance abuse in today's society is not that simple.

There are substances that can be abused for their mood-altering effects that are not drugs at all -- inhalants and solvents -- and there are drugs that can be abused that have no mood-altering or intoxication properties, such as anabolic steroids.

Generally, when most people talk about substance abuse, they are referring to the use of illegal drugs. Most professionals in the field of drug abuse prevention argue that any use of illegal drugs is by definition abuse. Those drugs got to be illegal in the first place because they are potentially addictive or can cause severe negative health effects; therefore, any use of illegal substances is dangerous and abusive.

Others argue that casual, recreational use of some drugs is not harmful and is merely use, not abuse. The most vocal of the proponents of recreational drug use are those who smoke marijuana. They argue that marijuana is not addictive and has many beneficial qualities, unlike the "harder" drugs.

But recent research has shown that even marijuana may have more harmful physical, mental, and psychomotor effects than first believed, and the National Institute on Drug Abuse reports that marijuana users can become psychologically dependent, and therefore addicted.

Illegal drugs are not the only substances that can be abused. Alcohol, prescription and over-the-counter medications, inhalants and solvents, and even coffee and cigarettes, can all be used to harmful excess. Theoretically, almost any substance can be abused.

For many substances, the line between use and abuse is not clear. Is having a couple of drinks every day after work to unwind use or abuse? Is drinking two pots of coffee in the morning to get your day started use or abuse? Generally in these situations, only the individual himself can determine where use ends and abuse begins.