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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.