Medical science says there is no question: Addiction is a disease.

Just consider what happens in the brain, says Dr. Mike "Mina" Kalfas, a certified addiction specialist in Northern Kentucky.

When heroin gets into the brain it acts like a key unlocking an intense euphoria. It stimulates receptors that trigger the primitive limbic system in the brain, which is below conscious control. It's where emotions live. Where instincts lie. And with that stimulation, the brain's level of dopamine rises. It's intensely pleasurable, Kalfas said, and when it falls, it's the opposite.

"The more the brain becomes used to having elevated dopamine from the drug, the harder it reacts to the falling levels of dopamine," said Kalfas. The addict seeks more. The fall is greater. The cycle is set.

When someone's withdrawing, their brain cannot work normally. They must have the drug to function – but can't function well with the drug.

"Getting off the drug is the process of a gradual return of brain chemistry to normal," Kalfas said.

Stop the drug and the user becomes emotional and their fight-or-flight mechanism is triggered. The body reacts with an onslaught of flu-like symptoms: Diarrhea, muscle cramps, goose flesh, runny nose, possibly fever and cough. "Even after that, there is a long period of post-acute withdrawal syndrome," Kalfas said.

The brain can take months, even a year, before returning to normal. "For many patients medications are necessary to get them through this period, or a relapse is imminent," Kalfas said.

And why are some people more susceptible to addiction than others? There's science in that answer as well.

"It's felt that about 20 percent of the general population has a 'mutated' mu receptor (which receives the heroin) that is abnormally, exquisitely sensitive to the process above," Kalfas said. "Even after one or two heroin uses, the system is so strongly primed that the obsessional drive to continue becomes overwhelming. Those with this mutation are often practically 'hooked' from day one."

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