Again, it is all about the dopamine.

All addictive substances send dopamine levels surging in the small central zone of the brain called the nucleus accumbens, which is thought to be the main reward center. Amphetamines induce cells to release it directly; cocaine blocks its reuptake; alcohol and narcotics like morphine, heroin and many prescription pain relievers suppress nerve cells that inhibit its release.

Addicts and first-time users alike get the high that correlates with the dopamine wave. Only a minority of novices, however, will develop the compulsion to keep taking the drug at great personal cost, a behavior that defines addiction.

Researchers now postulate that addiction requires two things. First is a genetic vulnerability, whose variables may include the quantity of dopamine receptors in the brain: Too few receptors and taking the drug is not particularly memorable, too many and it is actually unpleasant. Second, repeated assaults to the spectrum of circuits regulated by dopamine, involving motivation, expectation, memory and learning, among many others, appear to fundamentally alter the brain’s workings.

For instance, Dr. Volkow’s group showed several years ago that when cocaine addicts watched videos of people taking drugs, dopamine levels surged in the part of their brains associated with habit and learning, correlating with the intense drug cravings the subjects began to experience.

Her research and that of others has also shown that even after addicts are successfully detoxed and long clean, their dopamine circuits remain abnormally blunted. Substances that elevate dopamine levels in normal subjects had notably muted responses in ex-addicts.

This observation, experts say, may explain the intense difficulty addicts have staying clean, as the ordinary rewards of daily life may have little effect on the recovering brain. Only the drug of choice will send dopamine levels high enough for any kind of pleasure.

“Nora is as responsible as anyone,” Dr. Des Jarlais said, “for showing that addiction really does cause changes in brain function. Her work is a primary basis for considering it a disease, rather than poor choices or immoral behavior.”

Arming Doctors With Tools

Dr. Volkow has watched the microscopic events of addiction play out in fluorescent rainbow colors on brain scans for decades now. But those scans are used entirely for research, not for patient care. To the average doctor, she knows, the addict’s brain is impenetrable. All that is visible is irrational, illegal and sometimes threatening behavior. Surveys show most doctors prefer to keep their distance from addiction and addicted patients.

“My obsession is to engage the health care system in addiction,” Dr. Volkow said, to medicalize what is often considered mainly a criminal problem by arming doctors with their accustomed tools: effective treatments, even a vaccine.

“She’s been a champion of bringing addiction science into mainstream medicine,” said A. Thomas McLellan, director of the Penn Center for Substance Abuse Solutions at the University of Pennsylvania. Medicine is finally beginning to understand, Dr. McLellan said, that if you pay no attention to the behavioral factors leading to a chronic illness, be it diabetes or substance addiction, you can never catch up. “That’s been one of Nora’s big contributions.”

In a study published in 2000, Dr. McLellan pointed out that while the overall success rate for curing drug addiction with medications, therapy or both is not high (about half of treated individuals return to active substance use within a year), it is quite similar to overall successful treatment rates for other chronic medical conditions like asthma, diabetes and high blood pressure. Failure to take prescribed medications and backsliding to old bad habits is endemic, no matter what the condition.

But treating people with the prescription drug problems is particularly challenging, because, of course, for these particular drugs, physicians are the nation’s pushers.

The number of prescriptions written for potentially addictive pain medications has soared in the last decade, reaching more than 200 million in 2010, Dr. Volkow said. Surveys asking teenagers where they get pills find that relatively few buy from strangers. Many have their own prescriptions, often from dental work. Even more are given pills by friends and relatives, presumably out of other legitimate prescriptions.

Doctors may be flooding the country with narcotics, but most have never learned much about pain control. Dr. Volkow said that some data suggests that medical schools devote considerably less time to the subject than veterinary schools do. The Obama administration addressed exactly this deficiency in April with a call for doctors to undergo special training before being allowed to prescribe some of the most addictive painkillers.

“Students and residents have gotten the message that pain is undertreated,” said Dr. Mitchell H. Katz, an internist who directs the Los Angeles County Department of Health Services. “So they just prescribe higher and higher doses.” Meanwhile, he said, there is no evidence that treatment with opioids for more than four months actually helps chronic pain, or that higher doses work where lower ones fail. There is good evidence, however, that higher doses raise the risk of overdose and death.

“The dichotomy between good drugs prescribed by doctors and bad drugs sold on the street is just bad science,” Dr. Des Jarlais said. “N.I.D.A. is now having to address the fact that there are real problems with prescription drugs.”

The institute is starting a multipronged effort to teach and to learn more about pain control as it relates to addiction. The teaching part includes sponsoring performances of excerpts from Eugene O’Neill’s “Long Day’s Journey into Night” at medical conferences, in order to humanize for doctors the problem of addiction.

The learning part is continuing: New science for pain control may take some time to devise, Dr. Volkow said.

New Approaches