Substance Use Disorder in the Brain

Substance use disorder, commonly called addiction, is continued use despite harm, with craving, loss of control over the amount used, and a brain reward system that has reorganised itself around the substance.

Also called: Addiction, Substance dependence, Drug addiction, Alcohol use disorder.

ICD-10 F10-F19 · ICD-11 6C40

Wanting grows. Liking fades.

Most people picture it simply. Some drugs are so powerful that one try is enough, and after that the person is chasing the high forever.

The research shows something slower and stranger. The brain's learning system shifts its attention. It moves from the drug itself to everything that predicts the drug. The high shrinks while the pull grows. And life without the substance starts to feel worse than it did before.

Here are five findings. They run from the least settled to the most useful.

One try and you are hooked

Evidence: contested

This is the story told in schools and films. For most people, and most substances, it is not what happens.

A large American survey followed people who had ever used a substance. It asked how many went on to become dependent at any point in their lives.

For nicotine it was about two in three. For alcohol and cocaine it was about one in five. For cannabis it was about one in eleven. The picture shows a hundred people who tried each one, in that order.

So substances really do differ, and some are far riskier than others. But for most people, most of the time, trying something does not lead to addiction.

Several things decide who does get addicted. Genes account for roughly half the risk. Starting young matters. So do trauma, stress, mental health, and what else a person has in their life.

Getting wet in the rain

Everyone in a storm gets wet. Who catches a chill depends on how long they are out, what they are wearing, and whether they have somewhere warm to go.

Why the myth is sticky

It was built to scare people away from trying. It has a side effect. It makes the people who do develop a problem look like they chose badly once. In truth they were more at risk all along.

The signal moves to the cue

Evidence: established

Dopamine is not really about pleasure. It is about learning what comes next.

People call dopamine the pleasure chemical. It is closer to a teaching signal. It fires when something turns out better than expected.

In classic experiments, dopamine cells fired when a reward arrived. Then a light or a sound was played just before the reward, a few times over. After that, the cells fired at the light instead. They fired much less at the reward itself.

Every drug of abuse raises dopamine, directly or indirectly, and it does it every single time. So the cues around using become powerful triggers for craving. A place, a friend, a time of day, or a feeling can all become cues.

Researchers call this the difference between wanting and liking. With repeated use, the wanting grows. The liking stays the same, or it fades.

Your phone buzzing

The buzz makes you reach for the phone before you know what the message says. The buzz has hold of you, not the message.

Fewer places for the signal to land

Evidence: established

Brain scans show a quieter reward system in people with addiction.

Dopamine only works when it lands on a receptor. Brain scans show that people with addiction have fewer of one type, called D2, in the brain's reward centre. This is found in addiction to alcohol, cocaine, methamphetamine and opioids.

Fewer receptors means ordinary rewards register less. Food, company and a good day all land more weakly. The substance becomes one of the few things that still gets through.

Some of this comes first, before any drug use. In monkeys, the animals with fewer of these receptors were more likely to take cocaine. Then their social life changed and they rose in rank. Their receptors increased, and they took less cocaine.

After long periods without using, parts of the system do recover. It happens slowly, over months rather than days.

A radio with a weak aerial

Most stations come through as static. Only the strongest station is clear. So that is the one you end up listening to.

A new normal that is lower

Evidence: established

Over time, using stops being about feeling good. It becomes about not feeling bad.

The brain pushes back against anything that keeps shifting it. Each time a substance lifts the mood, the brain turns up the systems that pull the other way.

With repeated use, those opposing systems get stronger. The high gets smaller. The low afterwards gets deeper. And the resting level between uses sinks below where it started.

The brain's stress systems are part of this. Without the substance a person feels anxious, irritable and flat. Using brings them back up to something like normal.

This is why craving is strongest in bad moments. It is also why stress is one of the most reliable triggers for relapse.

Borrowing money you have to repay

Each loan feels like extra money today. The repayments come later, and they grow. In the end, most of what you borrow goes on paying back the last loan.

Other rewards still compete

Evidence: established

The most useful finding on this page, because it is what treatment builds on.

Addiction changes the brain. It does not remove choice. People with addiction still respond to what else is on offer.

In one experiment, rats had been taking cocaine for weeks. They were then given a choice between cocaine and sweet water. Ninety-four percent chose the sweet water.

In people, one of the most effective treatments is simple. It gives small, reliable rewards for each clean test. Having work, a safe place to live, and people who care also predict recovery.

The picture shows the idea. When using is the only reward in a life, almost everything flows that way. As other rewards grow, the flow splits between them.

A town with one shop

If there is only one shop, everyone shops there, however bad it is. Open a few more and people start going elsewhere. Nobody has to tell them to.

Why this is the useful one

It means recovery is not only about resisting. It is also about building a life that pulls harder than the substance does.

The same thing, without the vocabulary

Most people who try a substance do not get addicted. Four things decide much of who does. Genes, stress, starting young, and a person's circumstances.

With repeated use, the brain learns to want the substance whenever it sees the signs of it. Meanwhile the high itself fades. Ordinary pleasures register less, and life without the substance feels worse.

But people still respond to other rewards. Treatment works by weakening the pull, and by making the rest of life weigh more.