Drug tolerance:
It is a pharmacological concept describing subjects' reduced reaction to a drug following its repeated use. Increasing its dosage may re-amplify the drug's effects; however, this may accelerate tolerance, further reducing the drug's effects. Drug tolerance is indicative of drug use but is not necessarily associated with drug dependence or addiction.The process of tolerance development is reversible (e.g., through a drug holiday) and can involve both physiological factors and psychological factors.

Drug dependence
It is an adaptive state that develops from repeated drug administration, and which results in withdrawal upon cessation of drug use.A drug addiction, a distinct concept from substance dependence, is defined as compulsive, out-of-control drug use, despite negative consequences.An addictive drug is a drug which is both rewarding and reinforcing. ΔFosB , a gene transcription factor, is now known to be a critical component and common factor in the development of virtually all forms of behavioral and drug addictions, but not dependence.