The Argents developed a hollowed out bullet that contains Nordic blue monkshood. When shot with this bullet, a werewolf has 48 hours to live, presuming the shot wasn't in itself a killing blow. The wolfsbane acts as a poison, sapping the werewolf of their strength. As it spreads, black veins thread through their skin. Once the poison reaches their heart, they will die.
To counteract the toxin, wolfsbane of the same variety must be burned and the ashes pressed into the wound.