From my experience at Pensic XIII. You can have an allergic reaction to the spermacide in the sponge. I found that out the hard way. If ever there was a place where a rash shouldn't be, there it was.
So, try the thing while you're at home, and have easy access to cortisone cream. Don't take them on a trip, until you're sure you won't get a rash from the sponge.