I heard about this on the radio earlier. It is to prevent kids being born with spina bifida, amongst other things. How many? 250 a year, IIRC. OK. So why do I have to take it too? And everyone who has finished having kids or doesn't want to have them in the first place.
Mass medication. I can, though, see the argument for fluoridation of water, since we ALL have teeth (as far as I know)