Think about an experience that makes you feel good. It could be successfully completing a project at work, eating a warm chocolate chip cookie or taking a swig of whiskey. It could be a puff of a cigarette or a shopping trip. A dose of Vicodin or a hit of heroin.

Those experiences don’t automatically lead to addiction. So what makes a particular habit or substance an addiction? What propels some people to seek out these experiences, even if they are costly or detrimental to their health and relationships?

“Addiction is a biopsychosocial disorder. It’s a combination of your genetics, your neurobiology and how that interacts with psychological and social factors,” said Maureen Boyle, a public health advisor and director of the science policy branch at the National Institute on Drug Abuse. That means it’s a lot like any other chronic disorder, such as type 2 diabetes, cancer and heart disease. And just like other chronic diseases, addiction is both preventable and treatable, Boyle said, but added that if left untreated, it can last a lifetime. [Do Smokers’ Lungs Heal After They Quit?]

The mutual mechanism

Though everyone’s path to addiction is different — whether he or she tries a drug or a behavior because it’s what that person’s parents or peer do, or just out of curiosity — what’s common across all substance and behavioral addictions is their stunning ability to increase levels of an important chemical in the brain called dopamine, Boyle told Live Science.

Dopamine is a molecule that ferries messages across the brain’s reward center. It’s what gives people the feeling of pleasure and reinforces behaviors critical for survival, such as eating food and having sex.

When someone uses a drug or engages in a pleasurable experience, the same natural reward circuitry is activated. “The problem with drugs is that they do the job better than natural rewards,” said Dr. Hitoshi Morikawa, an associate professor of neuroscience at the University of Texas at Austin.

Different drugs tap into the dopamine reward system in different ways. Marijuana and heroin have a chemical structure similar to another neurotransmitter and can trick some brain cells into activating neurons that use dopamine. Cocaine and amphetamines, on the other hand, prolong the effect of dopamine on its target neurons, disrupting normal communication in the brain.

How quickly each drug can get into the brain, and how powerfully it activates neural circuits, determines how addictive it will be, Morikawa told Live Science. Some modes of use, like injecting or snorting a drug, make the drug’s effects almost immediate. “That’s why heroin, for example, is the last drug you want to take,” he said. “It’s very addictive.”

From experimenting to getting hooked

As individuals continue with addictive habits or substances, the brain adapts. It tries to reestablish a balance between the dopamine surges and normal levels of the substance in the brain, Morikawa said. To do this, neurons begin to produce less dopamine or simply reduce the number of dopamine receptors. The result is that the individual needs to continue to use drugs, or practice a particular behavior, to bring dopamine levels back to “normal.” Individuals may also need to take greater amounts of drugs to achieve a high; this is called tolerance.

Without dopamine creating feelings of pleasure in the brain, individuals also become more sensitive to negative emotions such as stress, anxiety or depression, Morikawa said. Sometimes, people with addiction may even feel physically ill, which often compels them to use drugs again to relieve these symptoms of withdrawal. [Booze Snooze: Why Does Alcohol Make You Sleepy, Then Alert?]

Eventually, the desire for the drug becomes more important than the actual pleasure it provides. And because dopamine plays a key role in learning and memory, it hardwires the need for the addictive substance or experience into the brain, along with any environmental cues associated with it — people, places, things and situations associated with past use. These memories become so entwined that even walking into a bar years later, or talking to the same friends an individual had previously binged with, may then trigger an alcoholic’s cravings, Morikawa said.

Brain-imaging studies of people with addiction reveal other striking changes as well. For example, people with alcohol-, cocaine- or opioid-use disorders show a loss in neurons and impaired activity in their prefrontal cortex, according to a 2011 review of studies published in the journal Nature Reviews Neuroscience. This erodes their ability to make sound decisions and regulate their impulses.

Risk factors

Some people are more susceptible to these extreme neurobiological changes than others, and therefore more susceptible to addiction. Not everyone who tries a cigarette or gets morphine after a surgery becomes addicted to drugs. Similarly, not everyone who gambles becomes addicted to gambling. Many factors influence the development of addictions, Boyle said, from genetics, to poor social support networks, to the experience of trauma or other co-occurring mental illnesses.

One of the biggest risk factors is age. “The younger someone is, the more vulnerable they are to addiction,” Boyle said. In fact, a federal study from 2014 found that the majority (74 percent) of 18- to 30-year-olds admitted to treatment programs had started using drugs at age 17 or younger.

Additionally, like most behavioral and mental health disorders, there are many genes that add to a person’s level of risk or provide some protection against addiction, Boyle said. But unlike the way in which doctors can predict a person’s risk of breast cancer by looking for mutations in a certain gene, nobody knows enough to be able to single out any gene or predict the likelihood of inheriting traits that could lead to addiction, she said.

You’ve probably heard of the brain’s reward network. It’s activated by basic needs — including food, water and sex — and releases a surge of the feel-good neurotransmitter dopamine when those needs are met. But it can also be hijacked by drugs, which lead to a greater dopamine release than those basic needs.

But the reward network isn’t the only brain network altered by drug use. A new review concluded that drug addiction affects six main brain networks: the reward, habit, salience, executive, memory and self-directed networks.

In 2016, a total of 20.1 million people ages 12 and older in the U.S. had a substance-use disorder, according to the National Survey on Drug Use and Health, an annual survey on drug use. And drug addiction, regardless of the substance used, had surprisingly similar effects on the addicted brain, said the new review, published yesterday (June 6) in the journal Neuron.

The review looked at more than 100 studies and review papers on drug addiction, all of which studied a type of brain scan called functional magnetic resonance imaging (fMRI).

More than half of the studies out there look at the effects of drug use on the reward network, said Anna Zilverstand, lead author of the new review and an assistant professor of psychiatry at the Icahn School of Medicine at Mount Sinai in New York City. [7 Ways Alcohol Affects Your Health]

“Because we showed that the effects are very distributed across the six different networks … [we can conclude that] an approach that only looks at one of these networks isn’t really justified,” Zilverstand told Live Science. “This [finding] will hopefully lead other researchers to look beyond the reward network.”

For example, the memory network is pretty much ignored in research on substance-use disorders, Zilverstand said. This network allows humans to learn non-habit-based things, such as a new physics concept or a history lesson. Some research has suggested that in people with substance-use disorders, stress shifts the person’s learning and memory away from the memory network to the habit network, which drives automatic behavior, such as seeking and taking drugs.

Another less-studied network is the self-directed network, which is involved in self-awareness and self-reflection, the review said. In people with addictions, this network has been associated with increasing craving.

Two other networks are involved in substance-use disorders: The executive network is normally responsible for goal-maintaining and execution, but drugs can alter this network as well, reducing a person’s ability to inhibit their actions. The salience network picks up important cues in a person’s environment and redirects the individual’s attention to them. (In people with drug addiction, attention is redirected toward drugs, increasing craving and drug-seeking.)

Which comes first, the brain activity or the drug use?

“For me, the most surprising [finding] was how consistent the effects were across addictions,” Zilverstand said. What’s more, “the fact that the effects are quite independent of the specific drug use points to them being something general that might actually precede drug use rather than be a consequence of drug use.”

Zilverstand said she hopes that more studies will look at whether some people have abnormal brain activity in these six networks naturally and if that activity just gets exacerbated if they begin drug use. It’s important to know if some of these traits precede drug use; if that’s the case, it might be possible to identify people who are prone to addiction and intervene before an addiction begins, she said.

Some research has pointed toward this possibility already. For example, studies have shown that some people have “difficulties … inhibiting impulsiveness before drug use,” Zilverstand said. “Some of these impairments precede drug use, and they may become worse with more drug use, but they exist before the problem escalates.”

The good news, however, is that activity in four of these networks — executive, reward, memory and salience — moves back toward “normal” once drug use ends. “We know that four of the networks (partially — not fully) recover but not yet what happens to the other two networks,” Zilverstand said in an email.

Zilverstand added that she’s particularly excited about an ongoing study called the Adolescent Brain Cognitive Development (ABCD) Study, which is tracking 10,000 children across the U.S. from around ages 9 or 10 to age 20 (the children are now around 13). Some of these individuals will inevitably become addicted to drugs, most likely marijuana or alcohol, Zilverstand said.

“We’ll be able to see if the effects that we found [in the review] exist in youth who have not yet abused drugs,” she said, and she predicted that researchers will be able to find a lot of the effects identified in the review in the six brain networks.

The authors noted that because some regions of the brain are very small — for example, the amygdala, which is found toward the center of the brain — the studies can’t identify strong signals from those areas on brain scans. So, it’s possible that drugs affect additional networks in the brain that are hidden because of the limitations of our technologies, Zilverstand said.

“We don’t want to conclude that [those effects] don’t exist,” she said.

If anything deserves to be called “the establishment view,” it is what Johann Hari — in his new book on addiction and the war on drugs, Chasing the Scream: The First and Last Days of the War on Drugs — calls the pharmaceutical model of addiction.

The pharmaceutical model says that addiction is about chemicals. Addiction is a chronic incurable disease of the brain. The brain’s pleasure centers are hijacked.

The pharmaceutical model may be the conventional wisdom, but it is certainly not without controversy. Researchers such as Gene Heyman and Bruce K. Alexander have long questioned whether the data support this picture.

As Hari reminds us, heroin use was rampant among U.S. soldiers stationed in Vietnam during that conflict. According to one study he cites, 20 percent of these American servicemen were heroin users. Over 85 percent of these users gave up heroin when they got home, the book says; they simply stopped taking the drug.

How do we reconcile this fact with the idea that the drug hijacks the brain and takes over? If that’s true, you wouldn’t expect a change of scenery (coming home from Vietnam) all by itself to break the neuro-chemical stranglehold. Alternatively, you might speculate that perhaps these heroin users were not really addicted. That the fact that they stopped proves this. But then we face a new problem: If the addiction is a chemical effect of the drug, then why weren’t they addicted?

Indeed, as Heyman claims in his book, there is good evidence that most addicts in the general population, as a matter of fact, eventually stop using drugs. They stop because they get to a point where they want or need or find that they are able to stop. They decide to stop. Now, this doesn’t fit very well with the conventional wisdom. You can’t simply decide to give up other diseases like diabetes or heart disease!

Or consider the fact that, as Hari explains, actual chemical dependence seems to be only a small part of drug addiction. If addiction were just about chemicals, then you would expect that the availability of nicotine patches — which can deliver smokers every bit of the nicotine they would get from a cigarette — would, in one fell swoop, eliminate the feeling that one needs to smoke. But not so. Only 17.7 percent of smokers using nicotine patches, according to Hari, break their addiction to smoking.

Hari also describes a fascinating reverse case. At some point in the 70s, heroin interdiction in Vancouver was so effective that there was virtually no heroin on the streets. This didn’t stop dealers from selling white powder and calling it heroin, and it didn’t stop addicts from hustling to get their hands on this powder so that they could shoot up. If it is the chemical itself that drives addiction and controls the addict, then you would have expected that Vancouver’s junkies, deprived of real heroin, would have been weened of their dependence.

That this didn’t happen is striking evidence — assuming the anecdote, originally reported by Alexander, is true — that not only is the chemical itself not sufficient to explain addiction, it isn’t even necessary.

Data like these suggest that addictions, although they no doubt interact with neural chemistry, can’t adequately be understood alone in neuro-chemical terms. And this is because it is people, not brains, that get hooked. To understand the actions of addicts, you need to look at their lives as a whole. When doctors claim, as they do, that addiction is a disease of the brain, they are saying something that is either trivially true (that the brain plays a role in addiction) or something entirely false (that the brain is the whole story).

It is true, as the old commercial made vivid, that a rat in a cage will forego water to self-administer morphine every time — and will continue to do so until it is dead. But, maybe, it’s not the morphine that best explains this but, rather, the fact that the sorry rat is locked up exposed, in complete isolation from its fellows, in an otherwise empty cage.

This observation led Alexander to wonder: Would a rat in a richer environment — one including not only water and morphine, but also other rats and good food and interesting landscapes — take the same self-destructive course of action? Or, rather, would this other rat find itself so totally dominated by the rewards that the drug alone provides? To investigate this, Alexander designed “rat parks” that were much larger, interesting and hospitable environments in which communities of rats were placed. He found that happy rats in happy rat parks behaved more like casual human drinkers than liked crazed addicts, in that they limited themselves to small amounts of morphine.

This result is striking — and it certainly corresponds to my own personal experience of addiction. I have known addicts, but I have known many more drug users who were not addicted.

Now, Hari is convinced that Alexander has unlocked the true essence of addiction. Addiction is caused by isolation. And the cure for addiction, it follows, is love. We need to give the addict back his or her feeling of connection to others.

This is a beautiful idea — and I like beautiful ideas. I also think that it gets something right. Addicts are disconnected and isolated. But we need to be careful in drawing a too-speedy conclusion. The fact that being isolated is a cause of addiction doesn’t yet clarify how sociality, love, friendship, isolation, etc., function in the lives of addicts. In particular, it doesn’t mean that you can fix addiction with love.

One reason love might not be all you need is that it could be that the wounds that lead us to turn to drugs, to really give ourselves over to drugs, might have their roots in our early lives. It’s hard to simply “get over” early childhood trauma. (Hari also celebrates the work of Mate Gabor, a clinician who argues, in ways that some thinkers worry might be a bit reductive, that the sources of addiction lie in trauma.)

But there is a deeper reason to worry that love couldn’t suffice. Whatever its causes, addiction would seem to be — something like this is Heyman’s view — a disorder of one’s ability to connect to others and value the things that human beings tend usually to value (such as food, exercise, sex, family, work). The remarkable and striking thing about many addicts is that they opt for self-medication over encounter — they turn inward and shut out the world.

It might be right, as Hari claims, that we would fix addiction if we could restore in the addict a sense of connection with the world around him or her, and with other people. But that’s not a prescription as much as it is a statement of the problem. Addicts are shut off.