What, exactly, is a "drug?" Given that definition, how do you feel about the morality of drug use? Under what circumstances do you think it is morally permissible? Under what circumstances is it morally wrong?

What, exactly, is a "drug?"  Given that definition, how do you feel about the morality of drug use?  Under what circumstances do you think it is morally permissible?  Under what circumstances is it morally wrong?  Give reasons for your claims and examples, if necessary.  

Remember to write at least 1 and a half pages, double spaced, and to cite all outside sources.