I was referring to the ability of the fluorine to stay attached the aryl ring -- it won't break off at any time inside your body.
So will this mean that you don't get any fluoride to your body from 4-FA? I'm very cautious with fluoride "derivied" drugs, as I don't want to get any excess amounts of fluoride to my body, Fluoride poisoning or Skeletal fluorosis is not something I'd like to risk.