If they continue their drug-seeking behavior despite of having family or relationship conflicts, financial difficulty, employment problems, health consequences or problems with law enforcement, chances are the person's drug use has advanced to the addiction stage.
Healthcare professionals, in trying to determine if someone is addicted to drugs, would probably use a diagnostic screening test, such as this drug abuse screening quiz.
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Source: National Institute on Drug Abuse. "Frequently Asked Questions." The Science of Drug Abuse and Addiction. Accessed February 2014