Drug or substance abuse is a compulsive act of using drug in spite of being aware of its adverse effects on health. Different drugs put different effects on human brain. All kinds of drugs impair the structure and functions of brain parts. Dopamine is a neurotransmitter that gets disturbed due to drug abuse. Drug affects the dopamine increasing the feelings of pleasure, as a result, person wants to use drug again and again. Therefore, drug abuse is referred to as brain relapsing disease. As the functions of brain get altered, people starting behaving differently in daily life. Drug abuse, by impairing brain functions, destroys the ability of people to think, to make decisions and to have control over self. Drug abuser becomes dependent on drugs and prefers drug over anything else he or she has in life. Craving of drugs impairs the sense of differentiating between right or wrong. Brain generates the solution of getting drugs rather than stopping it. It can be said that brain starts functioning in negative way. Human brain is associated with the cognition, behaviors, emotions and feelings. Once it functions negatively, people are overwhelmed by negative thoughts, feelings, behaviors and emotions. All these factors lead to dysfunctional personality.