Drug addiction is an abnormal and pathological condition, where you start taking drugs on a frequent and regular basis. The person is drawn towards a drug seeking behaviour and develops a vulnerability to relapse. In addition, the body develops an ability to respond to the naturally rewarding stimuli. Any chemical substances that the tissues of the body absorb, and that have an effect on the functioning of the body are called drugs; if the purpose is to regain health, it is fine, but when the person starts taking drugs unnecessarily, it is an issue of concern.
