Addiction is defined as the use of a substance whose continued use interferes with the responsibilities of life. Essentially, a person with an addiction to illicit drugs or alcohol cannot live a productive life because they need to do more drugs or drink more alcohol.
While controlling drug use is important, controlling the addiction is far more important. When controlling a drug, one may be able to stop the prescribing of it. That said, if a person has an addiction, he or she will do anything to obtain the drug of choice. Given the availability of drugs (Internet, clubs, drug swap parties, street sales), if a person wants something bad enough, he or she will find a way to get it.
Therefore, in order to control drug use, one must control the addiction. When one controls the addiction, the person addicted to drugs does not feel the need (once he or she has accepted treatment) to go out and get the drugs illegally.