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:
Note: I wrote くすり for illustrative purpose, but normally it is written as 薬.
このくすりを呑みなさい (therapeutic) スパイスはくすりにもなるのでたくさん食べましょう ("good for your body") 歩道でスケートボードしてて骨折か。まぁ、いいくすりになったんじゃないの？ (metaphoric) 水槽にくすりを入れてあげなさい (to non-humans)
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. Here it can more broadly refer to chemicals. If you are a chemist, a cleaner may well be a 薬品. If you are a layperson, to call an ordinal cleaner "薬品" sounds a bit odd. If it's some kind of an potent, special cleaning agent or something, then ppl use 薬品. I guess that's similar to English.
You might hear the word "ヤク", which I believe comes from 薬. This is exclusively a slang for entertaining drugs (basically illegal drugs).