There is a big difference between abuse and use.
In both cases a person is addicted.
This is a very easy thing to explain, but it is hard for people who are not users to understand.
Anyone using prescription pain killers for a month or more becomes addicted to the drug. It goes with the use.
A person who does not need a legal prescription and uses to get high is an abuser.
If Rush became addicted as a result of legal use he is not a abuser. The fact of addiction is not abuse in any way unless the purpose is not legitamate. It is a fact however, that sometimes legal addiction becomes abuse.
Why would Rush need to buy drugs on the black market if he had a legal presciption?