What the science shows is that the brain of an addict is fundamentally different from that of a non-addict. Initially, when a person uses hard drugs like heroin or cocaine, the chemistry of the brain is not much affected, and the decision to take the drugs remains voluntary. 

But at a certain point, a "metaphorical switch in the brain" gets thrown, and the individual moves into a state of addiction characterized by compulsive drug use.   These brain changes can persist long after addicts stop using drugs, which is why relapse is so common.  

Addiction should be approached more like other chronic illnesses, like diabetes and hypertension.  Drugs so alter the brain that addiction can be compared to mental disorders like Alzheimer's disease and schizophrenia.   Addiction is a "brain disease."

Today the brain-disease model is widely accepted in the addiction field.  Over the last decade or so, advances in brain-imaging technology have allowed researchers to measure the impact of psychoactive substances on the brain with increasing precision. 

Investigators have found that drugs like cocaine, heroin and alcohol increase the brain's production of dopamine, the neurotransmitter that regulates pleasure, among other things. This helps account for the euphoric high drug users feel. But these drugs deplete the dopamine pathway, disrupting the individual's ability to function.

At the Brookhaven National Laboratory on Long Island, for instance, Dr. Nora D. Volkow has found that even 100 days after a cocaine addict's last dose, there is significant disruption in the brain's frontal cortical area, which governs such attributes as impulse, motivation and drive. Dr. Volkow says that "the disruption of the dopamine pathways leads to a decrease in the reinforcing value of normal things, and this pushes the individual to take drugs to compensate." Other researchers have found a physiological basis for the craving so many addicts experience, but it is not yet clear how long such physiological changes remain.