When a person who uses drugs makes a conscious decision and repeated attempts to try and stop but cannot, this is called addiction. People do not start using drugs with the intention of becoming addicted, there is an over riding belief that they can control their intake even in the face of evidence that their usage has increased in amount and frequency.
Addiction is a brain-based disease, which impacts the normal functioning of Neurochemicals such as Serotonin, Dopamine and Norepinephrine, which is essential for mood stabilization. This is why addiction impacts the person Physiologically, Emotionally and Psychologically. Once a person becomes addicted it becomes all consuming resulting in chaotic unpredictable behaviour, volatile mood swings, which affects that persons family, friends, relationships and work.