That's a tempting definition of drug, but it's easy to dismiss because then the word would have no meaning. The accepted definition, from m-w.com: a substance other than food intended to affect the structure or function of the body.
Also, I'm not sure what your overall point is. Alcohol abuse still is considered dangerous now. (I say this as someone who drinks socially.) If you want to point out how cultural attitudes change how various substances are regulated, I don't think just "dangerous" expresses that.
That is a problematic definition as well. First, intended by whom? Is alcohol a drug if I drink it to get hammered, and not a drug if I drink it because I'm thirsty? Or is it the intention of the producer that matters, in which case how can any naturally occurring substance be a drug?
Looking past that, wouldn't hair gel or plaster used to make a cast for a broken bone qualify as a drug under that definition?
Maybe the word actually does have less meaning than you suggest. I would suggest the following definition: any substance included on Schedules I through V of the Controlled Substances Act.
Dictionary definitions are typically not scientific definitions in that they are colloquial and potentially ambiguous.
The definition implies that the substance has to enter the body somehow, so hair gel and plaster are out. As for intention, there's potential ambiguity, but for the most part we know what it means. Word definitions aren't programs, we can settle for "good enough." But the parents definition was, to me, obviously too inclusive.
Also, I'm not sure what your overall point is. Alcohol abuse still is considered dangerous now. (I say this as someone who drinks socially.) If you want to point out how cultural attitudes change how various substances are regulated, I don't think just "dangerous" expresses that.