It's not good for you, but it won't kill you or give you any nasty side-effects (other than what has already been felt). They formulated the therapeutic dose in-mind with this off-the-shelf stuff, in case people would abuse it (and often do).
They could have made a cough drop with a huge kick that would make you feel awesome from 1 drop. But a lot of people would OD on it, so they kept it weak and safe for the masses.
You should slow down though, if you keep doing 40/day for 2-4 more days, you'll probably start to get really woozy and vomit, just from all the menthol alone.
And yes, cough drops are quite tasty. Not afraid to admit when I was a kid, I would pop them like candy for fun too (5-10 a day, when not sick). Learned my lesson after having a few too many one afternoon, after a greasy lunch.