The scientific advice regarding silver amalgam fillings is that they are perfectly safe and offer many advantages over other types of fillings because of their lower cost, easy placement and durability.
However, as in all things in life, it is wise to use precautions when dealing with the very young even if the possibility of harm is remote. For your own peace of mind, why not postpone any type of non-urgent, operative dental treatment until you have finished breastfeeding. Having old fillings removed is more hazardous than placing new ones, because an aerosol containing amalgam particles is created by the dental drill. Other materials have disadvantages also, so it is preferable for a rapidly developing infant to have as little contact as possible with these types of substances
