How Addiction Works: Exploring Substance Use Disorders

By: Josh Clark & Sascha Bos  | 
Our understanding of addiction, including dependency on substances like heroin (shown above), has increased tremendously over the past three decades.
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Stories of the harmful consequences of addiction are common in our society. Reports of misuse of drugs like crack cocaine, heroin and alcohol abound, serving as cautionary tales.

­­There are many questions about the nature of addiction. Is denial a good indicator of addiction? Are some drugs as addictive as people say? There are even questions when it comes to addiction treatment. In order to persuade a person not to use addictive drugs, some people overstate the risks associated with psychoactive substances, but exaggeration can cause feelings of distrust.


Perhaps the best approach to the prevention of substance abuse is a clear, concise understanding of the process of addiction and the effects it can have on the user. To that end, the medical community has arrived at a concise and science-based view of addiction. We have learned much in the last few decades, but stigma remains.

What Is Addiction?

The National Institute on Drug Abuse (NIDA) defines addiction as "a chronic disease characterized by drug seeking and use that is compulsive, or difficult to control, despite harmful consequences."

The American Psychiatric Association (APA) prefers the term substance use disorder (SUD). SUD "is a complex condition in which there is uncontrolled use of a substance despite harmful consequences."


How Many People Have Substance Use Disorders?

According to the 2021 National Survey on Drug Use and Health, 46.3 million people (16.5 percent of the population) had a substance use disorder within the year. Alcohol use disorder was the most common SUD, affecting 29.5 million people.


How Substance Use Activates the Brain's Reward System

We learn to survive based on a reward system. When we do something that aids in our survival, like eating or exercising, our brain's limbic system rewards us for this behavior by releasing the feel-good chemical dopamine. Since we like how we feel, we learn to repeat the behavior.

Different substances affect the limbic system — the brain's reward center — in different ways, but most drugs cause the brain to release high dopamine levels. This release can be two to 10 times the amount usually present in the brain, giving the user a sense of a "rush" or "high."


The brain's reward system responds to substance use the same way as eating or exercising, but even faster and with more intensity since dopamine release is so much more significant. Since the amount of dopamine released is abnormal, the brain struggles to regain its chemical balance after a substance wears off, producing a hangover or withdrawal, which can manifest in physical pain or depression.

How Substance Use Changes Brain Chemistry

Over time, prolonged substance use can lead the brain to produce less dopamine. The brain's motivational center becomes reorganized, its priorities shuffled so that finding and using the substance (or another substance that will have similar effects) becomes a top priority for the brain.

Brain imaging studies show that continued substance use can cause physical changes in the areas of the brain associated with behavior control, decision-making, judgment, learning and memory. These research findings help explain why people with substance use disorders may continue using addictive substances despite wanting to stop.


11 Criteria of Substance Use Disorder

One criteria of substance use disorder is using more of a substance than originally intended.
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Health care providers use the "Diagnostic and Statistical Manual of Mental Disorders," 5th edition (DSM-5) to diagnose substance use disorder. The DSM-5 includes 11 criteria for SUD:

  1. Using more of the substance than originally intended, or using it for a longer period than intended.
  2. Wanting or trying (unsuccessfully) to cut back on use of the substance.
  3. Spending a lot of time on obtaining, using and recovering from use of the substance.
  4. Craving the substance.
  5. Failing to meet major obligations at home, school or work due to substance use.
  6. Continuing to use the substance despite its negative impact on interpersonal relationships.
  7. Reducing or giving up important recreational or social activities due to substance use.
  8. Using the substance in hazardous conditions.
  9. Continuing to use the substance despite its knowledge that it is causing or worsening a physical or psychological problem.
  10. Experiencing tolerance (the need for increased amounts of the substance in order to produce the desired effect). However, for certain substances — pain relievers, tranquilizers, stimulants and sedatives — tolerance is expected and may not indicate misuse.
  11. Experiencing withdrawal symptoms or using the substance (or a similar substance) to get over withdrawal symptoms. (This does not apply to hallucinogens and inhalants; like tolerance, withdrawal may not indicate misuse of pain relievers, tranquilizers, stimulants and sedatives.)

Doctors use these 11 criteria to determine whether a person has a mild, moderate or severe SUD; the more criteria a person meets, the more severe the disorder. Here are some tips for talking to someone about getting help from a mental health professional.


Why Do Substance Use Disorders Affect Some People and Not Others?

Substance misuse, once considered a moral failing, is now understood to have a strong genetic component. According to studies of twins, genetic factors may account for 40 to 60 percent of a person's risk of developing substance use disorders.

Mental health disorders like anxiety and depression, as well as environmental factors like peer pressure and whether family members misuse substances in the home, can also increase the likelihood of drug misuse. A person who is depressed, for example, may use drugs in an attempt to self-medicate.­ ­­


6 Types of Substance Use Disorders

Different categories of substances affect the body differently. And while it’s impossible to answer questions like, “Which drug is the worst?” we do have data on the prevalence and severity of substance use disorders for the most commonly misused substances, from alcohol to heroin.

1. Alcohol Use Disorder

Alcohol is readily available, socially acceptable — and incredibly dangerous. In 2021, 29.5 million people (10.6 percent of the population) in the United States met the criteria for an alcohol use disorder, making alcohol use disorder the most prevalent SUD in the U.S.


Most of these people (59.6 percent) had a mild disorder, while 19.5 percent had a severe disorder.

Despite alcohol’s widespread use (in 2021, 133.1 million people in the U.S. reported using alcohol in the preceding month), physicians consider alcohol withdrawal more dangerous than withdrawal from illicit drugs like heroin, due to the life-threatening physical symptoms of alcohol withdrawal.

2. Marijuana Use Disorder

Marijuana refers to the flower buds and leaves of the cannabis plant, which contains high levels of the psychoactive compound tetrahydrocannabinol (THC). Although there is promising research surrounding medical marijuana, the plant was associated with the second-highest rate of substance use disorder in the U.S. in 2021 (after alcohol).

Similarly to alcohol use disorder, most people with marijuana use disorder (57.6 percent) had a mild disorder, with only 16.1 percent meeting the criteria for a severe disorder.

3. Prescription Pain Reliever Use Disorder

In 2021, 8.7 million people in the U.S. misused prescription pain relievers, meaning they took pain relievers without a prescription, or they used their prescription medication in a way other than it was intended (for example, taking more than the prescribed dosage).

The largest share of people who misused pain relievers in 2021 (44.9 percent) got them through a friend or family member, while 39.3 percent of people who misused pain relievers got the medication through a prescription from their doctor.

Prescription opioids like OxyContin not only block pain receptors in the brain, they also release large quantities of dopamine, producing a sense of euphoria. Even when patients use prescription opioids as directed, they can develop a tolerance and experience withdrawal symptoms when stopping the drug.

The withdrawal symptoms may help explain why so many people struggle to stop using prescription opioids. In 2021, 5 million people in the U.S. (1.8 percent of the population) met the criteria for a prescription pain reliever use disorder. For 65 percent of those with a prescription pain reliever use disorder, the disorder was mild, and for 16 percent, it was severe.

4. Methamphetamine Use Disorder

Methamphetamine is available by prescription in the United States, but most people who use “meth” obtain it illegally. Methamphetamine is a short-acting, central nervous system stimulant that releases large quantities of dopamine into the brain.

Prolonged use of methamphetamine can cause hallucinations, paranoia, violence and potentially permanent changes to the areas of the brain associated with memory and emotion.

Unlike alcohol and marijuana use disorders, methamphetamine use disorder is more likely to be severe. In 2021, among the 1.6 million people in the United States who had a methamphetamine use disorder, 58.4 percent had a severe disorder, and just 17.4 had a mild disorder.

5. Cocaine Use Disorder

Cocaine is a stimulant drug derived from the coca plant (Erythroxylum coca). Coca is native to South America, and the indigenous people of Peru and Bolivia have used the plant for centuries, chewing on its leaves to stay alert and ward off hunger and thirst.

In 2021, 4.8 million people in the U.S. used cocaine, 1.6 million of whom met the criteria for a cocaine use disorder. In the U.S., dealers typically sell cocaine as a fine white powder, which is easy to mix with other substances, from cornstarch to the synthetic opioid fentanyl.

The prevalence of mild cocaine use disorder (42.2 percent) is about the same as severe cocaine use disorder (40 percent).

6. Heroin Use Disorder

Heroin is part of a class of drugs known as opioids, which includes prescription pain relievers like hydrocodone, oxycodone and morphine. Of the 9.2 million people who misused opioids in 2021, the vast majority (88.1 percent) misused prescription medication alone, while some (11.9 percent) used both heroin and prescription opioids, and a small percentage (5.7 percent) used heroin alone.

In 2021, 1 million people in the U.S. had a heroin use disorder. Like other opioids, heroin can be incredibly difficult to stop using due to intense withdrawal symptoms. Continued heroin use can lead to loss of the brain’s white matter, which is involved in behavior control and decision-making.


Behavioral Addictions

Although we often use the word "addicted" to refer to things we really like — think of the lyrics to "Toxic" by Britney Spears — the DSM-5 only recognizes one non-substance-related addiction: gambling disorder. Like substance use disorder, gambling disorder is characterized by an uncontrollable urge to gamble, despite the problems it may cause.

According to the APA, more research is needed on potentially addictive behaviors like eating, exercise, internet gaming, sex and shopping before they can be added to the DSM.


Substance Use Disorder Treatment

Geneticists have identified 59 genes significantly associated with addiction risk.
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Like other chronic diseases, there is no "cure" for substance use disorders; however, there are effective treatment options for managing SUD, including behavioral therapies and medications.

Genetic Factors

Genetics is among the most cutting-edge investigation being carried out today into the nature of addiction. A 2023 study identified 59 genes significantly associated with addiction risk. By identifying these aspects of addiction, researchers have formed the basis that could lead to the creation of drugs that treat specific addiction.


However, geneticists aren't betting that they will find a single gene that leads to addiction in people. Research so far has shown that genetic predisposition is most likely caused by a combination of genes working together.

Interestingly, research has shown that genes also play a role in making people less susceptible to addiction. Rather than simply not having genes that would make them more likely to become addicted to a substance, some people actually have genes that can keep them from ever becoming addicted.


There are already some medications in use that can treat addiction or alleviate symptoms. For many years, methadone has been used to treat heroin addiction. This drug affects the opiate receptors, and can alleviate both the psychological and physical withdrawal symptoms.

Another drug that has shown promise for treating heroin addiction, LAAM (levo-alpha-acetyl methadol), acts as an opioid antagonist, keeping opiate receptors in the brain from being stimulated, thus degrading the effect heroin has on the user.

The most widespread medications used to treat addiction are antidepressants. These drugs may address the feelings of despair that can result from psychological dependency. They can also help treat any preexisting condition, like depression, that may have led to the addiction in the first place.


Genes That Addict

Geneticists have isolated some genes and hormones that are related to specific types of addiction. Scientists hope that by studying these differences, we’ll be able to totally cure addictions someday.

  • Htr1b receptor gene: Mice that lack this gene like alcohol and cocaine more than those that have it.
  • Cnr1 receptor gene: Mice without it don’t respond as strongly to morphine.
  • ALDH*2 gene: Humans with two copies of this gene show less likelihood for developing alcohol use disorder.
  • Neuropeptide Y: Low levels of this hormone have been associated with desire for alcohol in mice.

[source: University of Utah]

Lots More Information

Related HowStuffWorks Articles

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