Drug Addiction is a chronic disease characterized by drug seeking and use that is compulsive, or difficult to control, despite its harmful consequences. It is defined as a chronic, relapsing brain disease.
Drugs affect the brain's reward system by producing an excess of dopamine, the chemical responsible for pleasurable feelings ( euphoric stimuli ). Our brains are wired to make sure we repeat rewarding activities, including those associated with drug use. Feelings of pleasure from drug use cause the brain to associate drugs with rewards, which causes cravings.
This process is usually referred to as THE PLEASURE CIRCUIT. In fact this reward / pleasure circuit starts overriding all our subconscious and conscious activities.