Before we can discuss what drugs do to the brain, we first need to know a few basics about how the brain works.

While the brain is an exquisitely complex organ, it can be simplified in to 3 main parts: the hindbrain, the midbrain, and the forebrain. They evolved in that order,  and develop in that order as we progress from fetus to adult.

The hindbrain is the most ancient, primitive part of the brain. It is responsible for respiration, circulation, and digestion; critical life functions which require no conscious effort on our part. Heck, even fish have hindbrains. 

The midbrain receives and integrates sensory information, and directs motor activity. The midbrain is involved in pleasure, reward, and motivation. When we do something pleasurable, this part of brain gets rewarded by a rush of dopamine. This makes us feel good, and motivates our behavior to do more of whatever it was that caused it to be released. Dopamine is the chemical that gets released when we eat/have sex /bond with the clan, activities important for our survival. 

The forebrain is the executive. It includes the prefrontal cortex, which continues to develop through adolescence and is only fully mature in adults. It is responsible for higher level functions – decision making, problem solving, strategizing. When someone tells you to be reasonable or use your head, they are talking to your prefrontal cortex. 

Usually the midbrain and forebrain work in concert. The midbrain might take in an image of a handsome guy, get a dopamine rush, and say, wow, he is super sexy and I would like to hook up with that man. The prefrontal cortex would then say, “Ahem, you are married”. The forebrain has our best interests at heart, though our midbrain may not agree. A well-developed forebrain can overpower our midbrain to determine our behaviors, but it cannot control our desires. You can avoid cookies as much as you want, but you are not likely to ever crave celery. 

So how do drugs affect the brain? 

Addictive drugs increase the number of reward-related chemicals in the brain, one of which is dopamine – the pleasure molecule. Whereas eating a good meal might give our midbrain a squirt of dopamine, certain drugs act more like a firehose. This chemical flood can result in feelings of euphoria, relaxation, and relief from stress. 

When drugs unnaturally increase dopamine in the brain (the chemical that normally gets released when we eat/have sex/bond with our clan) the user gets the message, “you don’t need food or sleep or friendships as much as you need this drug.” That is what causes some drug users to prioritize drug use over even their health and loved ones.

But then they develop tolerance: the amount they used to use no longer causes a release of dopamine. And in the meantime, in response to all the flooding from drug use, their brain has reduced its own dopamine production and sensitivity.

Fowler JS, Volkow ND, Kassed CA, Chang L. Imaging the addicted human brain. Sci Pract Perspect. 2007;3(2):4–16.

When they don’t consume progressively higher amounts of the drug, or when they stop using, this neurochemical deficiency precipitates withdrawal. And the symptoms of withdrawal – pain, terrible anxiety, vomiting, diarrhea, sweats and so on – reinforce the midbrain’s false belief that they need the drug in order to survive.

You can appeal the their reason, but remember, reason comes from our prefrontal cortex. Unfortunately, as if the changes noted above were not enough, chronic substance abuse is also associated with forebrain brain impairments, which reduces the user’s ability to regulate their behavior. When a person has a substance use disorder, the midbrain becomes the boss. 

So does that mean it is hopeless? That our friend/family member with substance use disorder is forever lost to their addiction? 

No. Our brain is powerful, but we are not powerless to affect it.

It’s a helluva lot of work, but we can rewire the pathways in our brain through prologued and repetitive practice of new behaviors.

However, even when people manage to rewire their brain and recover from their substance use disorder, they are at high risk of relapse. You know how a catchy song that you have not heard for over a decade can come on the radio, and suddenly you are belting along to every word? The brain can change so that new pathways predominate, and new behaviors ensue, but the old pathways are still there, dormant, and a single trigger can reawaken them. People with severe substance use disorder typically need to remain vigilant of triggers for the rest of their lives. 

Understanding the brain science behind addiction is important for eliminating the stigma associated with substance use disorder, particularly the assumption that people with this illness are bad/ weak/ making poor choices. Once affected, these people have no more choice in the matter than someone with diabetes does to make their pancreas start producing insulin properly again, or someone with kidney failure does to get off dialysis. Their brain is unwell, and they need compassion, support, and medical care.