It is difficult to establish how many people are affected by drug abuse. Drug users often do so in secret, and functional addicts may not be obvious to those around them. There is also an issue of definition—at what point is someone classified as an addict and at what point use becomes abuse are subject to debate. However, surveys have found that close to 10 percent of Americans age 12 and over use drugs, including unauthorized use of prescription medication, with 87 percent of them classified as addicts.
While the word "addiction" is often used to refer to a strong love or a habit, it has a narrower medical meaning. Drug addiction is a medical condition, and a drug addiction is more than just a drug use ritual or an inordinate fondness for them. Drug use certain can change the structure of the brain; in particular, the way it responds to stimuli. Eventually, rather than using the drug causing pleasure, the time between when the effects wear off and the next dose causes unpleasant physical and psychological symptoms, called withdrawal. At this point, the drug is used simply to eliminate these withdrawal symptoms.
Experts have noted that the concept of "drug abuse"—and the degree to which addiction is regarded as a problem—is tied into notions of "drug" and "abuse." Caffeine is, from a chemical and psychological perspective, a drug, and, from a neurological perspective, an addictive one. The feelings of being jittery and out of sorts before ones first cup of coffee are in part withdrawal symptoms. However, because it is legal practically everywhere, "caffeine addiction" is not regarded as abuse. On the other hand, regulated or illegal substances, such as cocaine, heroin, and crystal methamphetamine, are deemed drugs of abuse. Even marijuana, which is not addictive and generally considered less dangerous than alcohol, is considered subject to abuse, though alcohol is as well.