What Is Drug Addiction (Substance Use Disorder)?

Written by Keri Wiginton
Medically Reviewed by Shruthi N, MD on July 02, 2025
9 min read

Addiction is a disease that affects your brain, body, and behavior. It can make it hard to stop using a substance, even when it starts to cause problems in your life. The sooner you get help for addiction (also called substance use disorder), the better your chances of avoiding more serious health or life problems.

“We know that addiction is a chronic condition. And we know that people get better with treatment,” says Jeanette Tetrault, MD, professor of medicine and public health and associate director for training and education for the Program in Addiction Medicine at Yale School of Medicine.

Addiction doesn’t only involve illegal drugs such as heroin or cocaine. You can also develop a substance use disorder with legal substances such as alcohol, nicotine, sleep aids, or antianxiety medications.

Prescription pain medications — especially opioids — can also lead to addiction. You can get these drugs legally through prescription or illegally. Opioid misuse remains a major public health concern in the U.S. In 2018, opioids played a role in about two-thirds of all drug overdose deaths.

For many people, drug use begins as a choice. You might take something to feel better, ease pain, or manage stress. At first, you may feel in control. But over time, substances can change how your brain functions. These changes can last well beyond the drug’s effects and may make it harder to stop using — even when you want to.

Drug misuse is when you use legal or illegal substances in ways you shouldn’t. You might take more than the regular dose of pills or use someone else’s prescription. You may misuse drugs to feel good, ease stress, or avoid reality. But usually, you’re able to change your unhealthy habits or stop using completely.

Addiction is when you can’t stop. Not even when your health is in danger or it causes financial, emotional, and other problems for you or your loved ones. That urge to get and use drugs can fill up every minute of the day, even if you want to quit.

Addiction also is different from physical dependence or tolerance. In cases of physical dependence, withdrawal symptoms happen when you suddenly stop a substance. Tolerance happens when a dose of a substance does not work well enough over time.

When you use opioids for pain for a long time, for example, you may develop tolerance and even physical dependence. This doesn’t mean you’re addicted. In general, when narcotics are used under proper medical supervision, addiction happens in only a small percentage of people.

 

Addiction doesn’t have one single cause. It’s a complex condition that develops over time, shaped by a mix of biology, environment, mental health, and brain chemistry. Some people are more likely to become addicted than others, but it can happen to anyone.

Here’s what we know about the factors involved:

Genetics and family history. If addiction runs in your family, your risk is higher. Genes may account for about half of your chances of developing a substance use disorder.

Early drug use. The earlier you start using substances — especially during childhood or the teen years — the greater your risk. That’s because the brain is still developing and more exposed to change.

Mental health. Depression, anxiety, ADHD, and other mental health conditions are linked to higher rates of substance use and addiction. People may turn to drugs to ease emotional pain or feel more in control.

Trauma and stress. A history of trauma, abuse, or ongoing stress can raise your chances of misusing substances as a way to feel better.

Unstable relationships or homelife. Family conflict or lack of support may also increase your risk, especially during childhood.

Your brain is set up to make you repeat things that feel good. Drugs and alcohol tap into that system by causing a rush of dopamine — a chemical tied to pleasure, reward, and motivation.

At first, you might use a drug to feel good. You may keep using it to avoid feeling bad when it wears off. Everyday joys (such as food, hobbies, or time with loved ones) may start to feel dull in comparison.

Over time, you may need more of the drug to feel the same effect. And stopping can feel harder than expected.

“Addiction is a chronic condition that affects the brain’s reward system,” Tetrault says. “The alterations in this circuitry can lead to difficulty controlling use.”

Experts often describe the neurobiology of addiction as a cycle with three main stages:

  • Binge or intoxication — you feel high or a sense of reward from a substance.
  • Withdrawal or negative feelings — the high fades, and you feel low, anxious, or stressed.
  • Craving or preoccupation — you think about the drug and feel a strong urge to use it again.

Each stage affects a different part of the brain:

  • Basal ganglia, which helps with motivation and reward
  • Extended amygdala, which processes stress and withdrawal
  • Prefrontal cortex, which helps you plan, decide, and control your actions

Long-term substance use can change how these areas work. It doesn’t just affect pleasure. Ongoing drug use can change how you make choices, remember things, and manage stress. After a while, these brain changes can make you find and take drugs in ways that are harmful and beyond your control.

Addiction can affect your body, brain, emotions, and behavior. The signs may be evident, or they may build slowly over time. What starts as an occasional use can change into something harder to control, sometimes without you realizing it.

Doctors look for certain patterns when diagnosing substance use disorder. You may notice one or more of these symptoms:

Changes in use and control:

  • You have a strong urge or craving to use the drug every day or more than once a day.
  • You take more than you planned or use it for longer than you meant to.
  • You’ve tried to cut back or stop but haven’t been able to.

Life disruptions:

  • You keep using, even when it causes problems at work, school, or home.
  • You spend less time with friends or family and more time alone.
  • You spend a lot of your day getting, using, or recovering from the drug.
  • You skip basic self-care things such as eating, sleeping, grooming, or showering.

Risky behavior:

  • You steal, lie, or drive while drunk or high.
  • You have unsafe sex.
  • You keep using, even after it’s hurt your health or relationships.

Physical and mental effects:

  • You feel sick, shaky, anxious, or down when the drug wears off or you try to quit.
  • You feel less pleasure from things you used to enjoy.
  • You start using it just to feel “normal” again.
  • You feel more irritable, hopeless, or disconnected from yourself or others.

If these symptoms sound familiar, help is available. Treatment can ease withdrawal, support recovery, and help you take control of your life.

How to recognize signs of substance use disorder in loved ones

The signs of substance use aren’t always clear. Some people hide their use well or seem like they’re doing just fine. But even early on, changes in mood, behavior, or daily habits can point to a deeper issue. These changes often show up before physical problems.

“If they’re not doing the things they used to do, or they give up certain responsibilities to use a substance, that’s certainly something to be concerned about,” Tetrault says.

Here are some red flags to look for:

Loss of interest in hobbies or social events. They may stop doing things they used to enjoy or skip out on time with friends or family.

Neglect of responsibilities. This might include missing work, ignoring household tasks, or pulling away from close relationships.

Behavior or mood changes. Look for secrecy, irritability, anxiety, or depression. If they have withdrawal symptoms, these things can show up when they’re not using.

Loss of control over use. If they set limits to their substance use but don’t stick to them, that’s a sign they may be struggling.

Spotting these patterns early can make a big difference. So if something feels off, it’s worth bringing up with your loved one and suggesting they reach out for care.

Substance use disorder (SUD) is a medical diagnosis based on a set of well-defined criteria. Doctors use the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) to judge the symptoms and figure out the severity.

There are 11 possible signs of SUD. These include things such as strong cravings, using more than planned, failed attempts to cut back, and continued use despite problems at work, at home, or in relationships.

The diagnosis is based on how many of these criteria a person meets:

  • Mild SUD (two to three symptoms)
  • Moderate SUD (four to five symptoms)
  • Severe SUD (six or more symptoms)

Your life doesn’t have to fall apart to suggest you have SUD. Even if you haven’t lost your job or relationships, ongoing cravings or failed attempts to quit mean it’s time to get help.

“What I don’t want is for people to think, ‘Well, I haven’t gotten a DUI or lost my job, so it must not be a problem for me,’” Tetrault says. “If they notice changes to their interaction with substances — like having a strong urge to drink alcohol at noon on a Tuesday — that’s worth paying attention to.”

A full diagnosis should come from a health care provider. But noticing the early signs can help you act sooner, which can make recovery easier.

Most people who take their pain medicine as directed by their doctor do not become addicted, even if they take the medicine for a long time. Fears about addiction should not prevent you from using narcotics to ease your pain, but it’s smart to use caution.

But if you’ve misused drugs or alcohol in the past or have family members who have, you may be at a higher risk.

To avoid pain medicine addiction:

  • Take the drug exactly as your doctor prescribes.
  • Tell your doctor about any personal or family history of drug misuse or addiction; this will help them prescribe the medicines that will work best for you.

Remember, it’s common for people to develop a tolerance to pain medication and need higher doses to get the same level of pain relief. This is normal and is not a sign of addiction. With addiction, you may need to use higher doses, but it’s not for pain relief. Still, talk to your doctor if this effect becomes troubling.

When you stop or cut back on drugs or alcohol, your body and brain may react. This is called withdrawal. It can be mild or severe. It depends on what you used, how much, and for how long.

Mild symptoms might include feeling anxious or shaky or having trouble sleeping. More serious symptoms, such as seizures or confusion, can happen with alcohol or certain medications like benzodiazepines. This kind of withdrawal may need treatment.

“Alcohol withdrawal can be serious and the severity can progress pretty quickly,” Tetrault says. “In many cases, medical monitoring is necessary.”

Opioid withdrawal is rarely life-threatening, but it can still be “extremely uncomfortable,” Tetrault says. You may have muscle pain, chills, nausea, or trouble sleeping. These symptoms can make it hard to stop using, even if you want to.

But doctors can prescribe medications to ease withdrawal and help you safely cut back or quit. You might need to get care in a clinic, at home with supervision, or in a hospital. The most important thing to know is you don’t have to go through it alone.

“Treatment of withdrawal should be considered part of a comprehensive care plan for patients with substance use disorder,” Tetrault says. “Withdrawal treatment alone is generally not sufficient for establishing stability.”