Been using that as needed for years. If my timing is right, occasional use intercepts the pre-cancers before they become cancers. Other times, I am too slow to react, and off to my scalpel-happy dermatologist I go.
Perfect timing, for me at least, means mission accomplished with little or no skin-level discomfort. If my timing is imperfect, as it so often is, I do get some itching, (minor) burning, and flaking from what I guess is knee-jerk overdosing.
I have never had any systemic reaction to its use.
Fwiw, skin cancer was said to be Reg Park's cause of death, and I haven't had that either ... so far.