JFK is a textbook example of an arguably functioning user. He received daily injections of amphetamines, administered by his physician. Some historians believe this addiction was the reason for several major screwups, in particular his meeting with Kruschev that almost ignited WW3.
You call that a screw up? So much for the virtue of peace talks. There's a big difference between legitimate consumption with guidelines and 'helter-skelter its-up-to-you kind of drug use, though im sure many would debate me on that.
Functional addicts...many people addicted to tobacco, because yes, they are functional and yes they are (quite strongly) addicted.