Well I've seen it described by one user as "a hot coal ember in my stomach that wouldn't go out." It's only anecdotal though.
Said user panicked, went to the hospital, and survived. I won't name them though for privacy sake.
I'm not trying to fearmonger I just wanted to give a worst case scenario type thing. There have been other users who said it really wasn't all that bad, again anecdotal at best.