Is there a difference between drug use and abuse?

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This is a question I have had for some time now. I have heard both these terms used very frequently, and I wonder, is there a difference? Where is the line drawn between the two? If you use illicit drugs, does that automatically mean you participate in drug abuse? We all see what are known as "crackheads" running the streets. Haven't bathed in weeks, probably haven't eaten in longer. Faces drawn and sallow, ever searching for the next hit. These people are drug abusers, no doubt. But I used to know a person who did crack on a recreational basis. He kept a job, house, paid his bills on time. He was always clean and looking nice. If he had money left over after what needed to be taken care of was, then he would consider indulging in his habit. Is he an abuser too? Is there a line between drug use and abuse? And where is it?