Don't zap with fillings?

I was thinking of buying a zapper, but i see some people mention on here that you shouldn't zap if you have metal in your mouth. I only have 1 cavity filling (lucky me, lol), so it's really not a large amount of metal. So, should i zap now, or wait until i remove the filling? Can't you cover up the filling while you zap, like with a piece of gum?