How about this? The nails are made using a special process, which involves forging them of Cold Iron (by the way, some people, including myself, believe that "Cold Iron" refers to the way it was shaped, IE by cold forging it, instead of it being a special kind of metal, just a fun fact), then soaking it in water infused with wolfsbane, mistletoe berries, and hawthorn wood. The reaction is more scientific than magical in reality, that combination just happens to be poison to them.