If someone continues to use a drug or keeps engaging in some kind of risky behavior after losing relationships, health, and/or employment, or getting in legal trouble, it can feel baffling. We wonder: If it’s causing pain and suffering in their life, why don’t they simply stop doing it? Many people will look at the situation and think the person really ought to make more of an effort—that addiction reflects a person’s weak character or that they simply aren’t adept at making good choices.

But modern psychology and neuroscience understand addiction differently and have come to classify it as a mental disorder. This is because brain imaging shows that, over time, addiction actually changes brain structure and function, specifically in areas controlling reward, stress, and self-control. In other words, changes in the brain and body turn voluntary drug use or risky behaviors into a chronic, compulsive brain disease. These brain changes affect motivation, reward, learning, self-control, and behavior, which is why the person’s behavior persists despite serious consequences. Science, in other words, has fundamentally changed how we understand addiction, and why we now call it a mental illness.

What Is Mental Illness?

It might be helpful to back up and explain how we classify something as a mental illness. The American Psychological Association defines a mental illness as an impairment or disturbance in thoughts, feelings, or behaviors that causes distress and/or impairment. Prolonged substance abuse and repetitive engagement in destructive or risky behaviors meet these criteria because the person has lost control over their behavior, it impairs their thinking and emotions, and they are continuing to use or engage in the behavior despite the harm it’s causing them and others.

Addiction as Mental Illness

Addiction is officially understood as a chronic mental disease of brain reward, motivation, memory, and related circuitry. It affects dopamine learning and executive functioning, contains cravings, and is habit-forming. An individual with obsessive-compulsive disorder (OCD) might repeatedly lock and unlock doors to regulate their anxiety and stop the intrusive thoughts, even if they know rationally that action isn’t actually going to keep them any safer. A person suffering from depression might repeatedly crawl into bed even if they have read the research on the importance of exercise in combating depression. When our brain is malfunctioning, we are sometimes powerless to stop or start an action that isn’t rational. This is where it’s helpful to understand it as a disorder: Similar to the person with OCD or depression, when someone is in addiction, their brain isn’t able to think rationally. Their decision-making is impaired. When they try to halt the substance use or behavior, it creates distress, and, thus, their brain has become rewired—their behavior has become compulsive rather than based on a decision.

This is why, while many people may say, “This person suffers from addiction and mental illness,” treatment providers will say, “This person is suffering from two mental illnesses.” Addiction is one of them.

Why Does It Matter?

Why does calling addiction a mental illness matter? For several reasons. First, it reduces moral blame—not only is it inaccurate to call addiction a weakness, but it’s also entirely unhelpful. Second, it encourages treatment—people are more likely to seek the help they need if they understand how difficult it can be to “just stop.” Additionally, understanding it as a mental illness can improve public policy and therefore access to needed treatment. Finally, it recognizes that addiction is, in fact, treatable. It’s an illness that can be helped.

Understanding It as a Mental Illness Is Not the End of the Story

All of that said, it’s important we understand that just because addiction is a mental illness, it doesn’t mean people should not be held accountable for their behavior. Brain changes help explain behaviors, but they don’t excuse them or remove responsibility. Understanding biology improves treatment, but recovery still requires a decision for someone to make that they need to make a change and hard work to make it happen. The question is not “Why won’t they stop?” It’s “Why is stopping so difficult—and how can intervention and treatment help rewire the brain so different choices can be made?”