I've never suffered from it personally but possible effects 'could' be -
Very hot drip tip due to sub ohming etc'.
Letting someone try your gear who's a scabby shit.
Not cleaning the drip tip.
Allergy towards SS.
A bit of high nic juice on the end of the drip tip.
An allergy towards PG.
You're getting a cold sore coming.
Can't think of anything else it can be.