The brain is the most complex organ in the human body. It regulates basic functions, enables us to interpret and respond to experiences, and generates our thoughts, emotions, and behaviors.
Alcohol and drug use can affect important areas of the brain that control motivation, impulse control, reaction to stress, memory, and decision-making, and can eventually lead to the compulsive substance-seeking and use that is central to the experience of substance use disorders (SUDs).
In the early 1990s, scientists began to understand how repeated substance use affects the brain. Brain scans showed that, as is the case with other brain disorders, SUD affects tissue function in two main parts of the brain: the limbic system and the cortex.
The Limbic System
The limbic system, located deep within the brain, is responsible for our basic survival instincts: eating, drinking, finding shelter, having sex, and caring for our young.
When we engage in these essential tasks, our brain reinforces the behavior with the release of dopamine--our reward for surviving. Dopamine and other brain chemicals are also transmitted to the amygdala and hippocampus, which record a memory of that feeling so we seek it again.
Substance use activates this dopamine process in the survival center much more powerfully than natural rewards like food or sex. And when substance use is repeated, it can hijack the limbic system to make the brain believe that it needs the substance in order to survive. Over time, more and more of the substance is needed to activate the same level of reward (also known as tolerance), which also causes the brain’s circuits to become increasingly imbalanced.
The Prefrontal Cortex
The prefrontal cortex regulates our decision-making and impulse-control--both of which distinguish us from other animals. The fact that this part of the adolescent brain is still a work-in-progress until a person is in their early 20s puts young people at high-risk for making poor decisions--such as trying addictive substances in the first place or continuing to take them. It also means that using drugs or alcohol during this period of development may cause changes to brain function that have profound and long-lasting consequences, including an increased risk for developing an addiction.
At any age, from adolescence into adulthood, chronic exposure to alcohol or drugs can disrupt normal brain function. Just as continued use may lead to tolerance--or the need for larger amounts of alcohol or drugs to produce an effect--it may also lead to a SUD, which can drive a person to seek out and use substances compulsively despite negative consequences. As SUDs progress, they change the brain in ways that erode a person’s self-control and ability to make sound decisions while simultaneously producing intense impulses to seek and use alcohol or drugs again. This is what it means when scientists say that addiction is a brain disease.