The definition I like from Google is "the misuse of chemicals that can alter the functioning of one's body and mind." It's substance abuse because the substance in question generally has an express purpose, and so you're abusing it if you're using it for a different purpose. Painkillers would be a good example: they are substances which exist to dull or numb pain, but you can abuse their purpose and instead take them for a high.
Mmaestro
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"You're never more alone than when you're alone in a crowd"
-Captain Sheridan, Babylon 5
Music of the Moment: Radiohead - In Rainbows