What Makes Addiction A Disease?Q & A

Addicts’ brains are physically different at the cellular and molecular level than the brains of casual drug users. These differences cause the brain to malfunction, altering the way addicts think and behave. Neuroscientists say that in addicts, normal balancing mechanisms go haywire, allowing instinct-like circuits that find drugs rewarding to hijack the brain, resulting in compulsive drug use.

As Science Magazine put it in a 1997 article, “The addicted brain is distinctly different from the non-addicted brain … That addiction is tied to changes in brain structure and function is what makes it, fundamentally, a brain disease. A metaphoric switch in the brain seems to be thrown as a result of prolonged drug use.”1

These differences occur in the brain’s Limbic system, the network responsible for feeling pleasure and both positive and negative emotions. Drugs including alcohol, heroin, cocaine, methamphetamine and marijuana greatly overstimulate this network — by as much as ten times that of natural rewards2 — to produce a high.

For most people, alcohol and drugs are pleasurable but not life-changing. Most don’t get addicted. However, for others, like those who may have a genetic predisposition for addiction, drug-induced over-excitement of the Reward System causes profoundly positive emotional effects: the perception that drugs are the solution to their deepest emotional problems. That feeling motivates them to take more and more drugs to continue to access their solution. But excessive drug use can flip the “metaphoric switch,” permanently altering the brain cells in this system.

The “switch” is the point-of-no-return from recreational drug use into the compulsive use of addiction. It’s triggered by the process of increasing drug tolerance, which requires more drugs to achieve the same high (ie. the same solution) one used to get from less. In this process, the Reward System changes its normal balance, going from treating occasional drug use as an anomaly, to expecting drug-induced overstimulation to be a permanently “new normal” environment. (For more on how tolerance develops, click on How Tolerance Changes the Brain.) 

Once the “switch” is thrown, the Reward System requires continuous over-excitement. It shifts from merely liking drugs to needing them to maintain the over-stimulation required by its “new normal.” Without drugs, the brain experiences a stimulation deficit and plunges into withdrawal. Users have to continue to take drugs to avoid that terrible fate rather than just to get high, and drug use becomes compulsive.

The Reward System’s “new normal” also shift the customary balance between emotional and intellectual brain systems. Ordinarily, these systems are equally balanced contestants in decision-making. But in addicts’ brain’s, the abnormally-excited Reward System gains an edge over corresponding rational systems. As a result, addicts over-value the short-term emotional need for drugs and under-value the rational consequences of drug abuse.

In short, drug-induced molecular rewiring alters addicts’ Reward System brain function, skewing their thought processes. The imbalances that result, both within the Reward System and over rational brain systems, explains why addicts use drugs obsessively despite worsening consequences that would convince people with properly balanced brains to stop. 

For more click on What Part of the Brain is Altered By Addiction?

Click here to return to the Questions About Addiction Menu.


1. Addiction is a Brain Disease, and It Matters, Science Magazine, Oct. 3, 1997.

2. National Institute on Drug Abuse:  http://www.nida.nih.gov/scienceofaddiction/brain.html.


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