I took selenium while I still had my mercury fillings and it certainly made me feel grungy so it must be a detoxifier ie pulls mercury out of your body BUT if you still have mercury fillings it perhaps can pull the mercury right out of your fillings into your bloodstream, making you feel even worse. When I stopped taking it, I stopped feeling grungy.
You are not supposed to take ANY detox product UNTIL you have had ALL your mercury fillings out because it may start excess mercury more than you already have circulating through your body, making you worse. Wait until you have them all out before you start taking it again; this is what I have read and it certainly proved correct for me.