A person starts out as an occasional drug user, and that is a voluntary decision. But as times passes, something happens, and that person goes from being a voluntary drug user to being a compulsive drug user. More than anything else, drug addiction is a character flaw.
Drug addiction is a brain disease.
Every type of drug of abuse has its own individual mechanism for changing how the brain functions. And these changes have a huge influence on all aspects of a person's behavior. The drug becomes the single most powerful motivator in a drug abuser's existence. He or she will do almost anything for the drug. This comes about because drug use has changed the individual's brain and its functioning in critical ways.