Both Uses

caustic
in
A Gesture Life
(Edited)

There is an almost caustic discomfort in my lower chest and then down my right arm and right leg, what feels like lines of internal burns that sharply prickle and itch and ache.
p. 68.1 *

caustic = harsh
I say mercifully because it was never my nature to harbor such thoughts, which have always been near-caustic to me, but in respect to the doctor a vital, searing charge was propelling me, an ashen, bitter hate whose taste I no longer abhorred.
p. 263.3 *

Definitions:

(1) (caustic as in: a caustic chemical)
damaging or harsh -- often of a corrosive chemical like a strong acid
(2) (caustic as in: a caustic person)
sarcastic, critical, or harsh
(3) (meaning too rare to warrant focus)