I saw on a board the other day someone referring to insecticide
as "薬", which doesn't really fit with the English conception of
"medicine". I looked up "薬" in Kenkyuusha, and sure enough,
"chemical" was listed as a meaning (along with "enamel", which
is just weird if you ask me, but anyway . . .). My question is,
how broadly does the "chemical" sense apply in normal usage? For
example, would an adhesive,a cleanser, or a solvent be classed
as "薬"? Additionally, are there other senses which might
surprise an English speaker? If you pronounce it くすり, then it
basically exclusively refers to something that will have a
positive effect on living beings (primarily therapeutic but also
in a more lax manner). In a strict sense くすり will be material,
but it can also be used as a metaphor: Metaphorically, it can be
applied to non-living things as well, but it must have an
positive effect: 日本経済に処方するくすりとなるか？ When it's pronounced やく, it's
basically exclusively used in conjunction with other kanji, as
in 薬品、爆薬、薬量、薬価 etc. etc.