Final answer:
The behavior characterized by rationalizing substance use and shifting blame associated with a substance use problem is known as denial, which is a typical defense mechanism used in cases of drug addiction.
Step-by-step explanation:
The behavior of attempting to rationalize substance use, minimizing the harmful effects, deflecting attention from one's own problem to something else, and blaming childhood experiences for a substance use problem is characterized by denial. This is a common mechanism that those with addiction issues utilize to justify their continued use of drugs and to avoid confronting the negative consequences that accompany their drug addiction. Denial can involve a range of behaviors, such as lying about or hiding substance use, making excuses, blaming other factors for one's usage, or underestimating the amount and effects of the substance taken.