What's substance abuse or chemical dependence?

Maladaptive pattern. Substance abuse, also known as drug abuse, is a maladaptive patterned use of a substance (drug) in which the user consumes the substance in amounts or with methods not condoned by medical professionals. Substance abuse/drug abuse is not limited to mood-altering or psycho-active drugs. Activity is also considered substance abuse when inappropriately used (as in steroids for performance enhancement).
Addiction. Substance abuse is the act of using a drug, both illegal drugs and prescription drugs, for purposes other than their intended use. Chemical dependence occurs when in the course of substance abuse a person develops tolerance (the need to take increasing amounts to get the same effect) and dependence (developent of withdrawal symptoms when one stops).