This question exists due to ignorance as to what electricity is and the rules that govern it. The electric current from a Blood Electrifier is electron flow which takes the path of least resistance. It travels from one electrode to the other. It does not enter the body and then spread to all parts of the body like a medicine would. Usually the shortest path between the electrodes is the path the current takes. But some minority of electrons stray away from this "highway". The farther their alternate path "bows" away from the main path, the more fluid and tissue has to be travelled across which means the amount of resistance for that path is more. The formula for current is I=V/R so that the greater the resistance, the lesser the resultant current. Subsequently the farther the alternate path is from the main one, the more insignificant the amount of electric current becomes. If you are using the electrodes wrist to wrist (as you would do if you are in the know) then the majority of the current passes above the heart through the arteries there and also through lymph fluid. The amount of stray current that would go up the neck and extend into the jaw is infinitesimally small and completely unworthy of any consideration at all, especially in regard to tooth fillings.
Side note: You can kill a tooth infection using one electrode on top of the tooth and the other one under the jaw directly under the tooth. Turn up the current high enough so that it disturbs your visual perception. 1-5 minutes twice a day till pain subsides. Then go to a dentist and fill the cavity with ceramic. Otherwise bacteria will keep entering it and reinfecting it.