Classical conditioning in

• drug tolerance (increasingly larger amount of drug is needed to attain same effect)
• withdrawal symptoms (effect of not getting the drug once addicted)
• paradoxical overdose (overdose resulting from a normally tolerated amount of the drug)

US: drug administration (e.g., heroin)
UR: effect of drug (e.g., pleasurable sensations; sense of euphoria and well-being; constipation)

CS: stimuli present in context and environment at time of drug administration, which always precede the drug US (e.g., consistent particular room, furniture, people, time of day)
CR: result of body's attempt to compensate for UR with responses opposed to the UR (e.g., pain, aches, cramps; sense of anxiety, depression, paranoia; diarrhea)

Drug Tolerance: CR opposes UR; increased strength of learned compensatory CR to the contextual CSs accompanying drug administration (thus requiring larger amounts of drug US to produce a stronger UR to counter the CR)

Withdrawal: CR without UR; effect of compensatory CR to contextual CSs when drug US is not provided (and thus no UR occurs)

Paradoxical Overdose: UR without CR; effect of previously tolerated large drug dose's UR when context changes and CSs are not present (thus providing no compensatory CR to counter the strong UR)