As a direct treatment for whitespot it does as chrismr said squat diddly do, but its "theraputic(sp?) effects" can help. Personally other than a bit melafake (homemade melafix) I never use meds for whitespot, I just up my tank temp to 32degrees C for 3-7 days (untill all signs have gone plus a day or two extra). Most fish can deal with these temps for this period of time without any probs, only thing to watch is oxygen levels.
Andrew