Coffee

It's the most corrosive thing I consume.   It feels like it is scouring your insides, and yet by all accounts it is not bad for you, at least when consumed in moderation.

Alcohol feels more like a diluted hydrochloric acid solution, and one can imagine it doing a lesser scouring as it goes down.  But its ways are more insidious, like those of the discreet poisons used by spouses who want to murder, quietly, in baby steps over a long time.