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No one is totally resistant to addiction, although “some develop it more easily than others,” says David McCann, chief of the medications discovery and toxicology branch at the National Institute on Drug Abuse in Rockville.

“By learning about how brain systems are affected in all animals and people, we identify targets to pursue for medication,” explains Dr. McCann, who works on animal models, studying especially cocaine and methamphetamine dependence. “It’s really important to follow genetic differences, but we have to follow many paths.”

One of the ways that cocaine changes the brain is to affect a pleasure-enhancing neurotransmitter called dopamine.

“Scientists have found that a different neurotransmitter, called GABA, has an opposite effect on dopamine,” Dr. McCann says. “One of the drugs of promise regarding treatment is vigabatrin, which boosts brain levels of GABA, fights the effect of cocaine on dopamine, and reportedly decreases craving for cocaine.”

It also has been shown to be effective in blocking anxiety — an important discovery, Mr. McCann points out, “because stress often is reported as a triggering event sending people back to drugs such as cocaine.”

While there has been a lot of progress in understanding the brain and addiction, Mr. O’Brien finds progress in delivering better care hampered by “not enough trained physicians who can deliver this care.”

He is aware of just one medical school — the University of Pennsylvania — that makes compulsory a full course on addiction.

Complicating the therapeutic picture is the fact that no single treatment works for everyone.

“A good therapist must be flexible and able to tailor treatment to the individual and have knowledge of scientific literature,” Mr. O’Brien says. He believes in “talk therapy” — often called cognitive behavior therapy — but says “the best results occur with medication.”

“Drug addiction treatment does work,” declared Mr. Leshner in his talk. “It is a biological illness just like others. So the ‘war on drugs’ is the worst metaphor. Simply taking the criminal justice approach doesn’t work. If we treat addicted offenders, they won’t come back. That is the truth. Addicts put in jail have about a 70 percent chance of being arrested again if they are not treated; when treated, flip the numbers.”