Silver bullet
In folklore, a bullet cast from silver is often one of the few weapons that are effective against a werewolf, witch, vampire, or other monster. The term is also a metaphor for a simple, seemingly magical, solution to a difficult problem: for example, penicillin was a silver bullet that allowed doctors to treat and successfully cure many bacterial infections.
In folklore
Some authors asserted that the idea of the werewolf's supposed vulnerability to silver dates back to the Beast of Gévaudan, a man-eating animal killed by the hunter Jean Chastel in the year 1767. However, the allegations of Chastel purportedly using a gun loaded with silver bullets are derived from a distorted detail based primarily on Henri Pourrat's Histoire fidèle de la bête en Gévaudan (1946). In this novel, the French writer imagines that the beast was shot thanks to fictitious medals of the Virgin Mary, worn by Jean Chastel in his hat and then melted down to make bullets.
In the Brothers Grimm fairy-tale of The Two Brothers, a bullet-proof witch is shot down by silver buttons, fired from a gun.
In some epic folk songs about Bulgarian rebel leader Delyo, he is described as invulnerable to normal weapons, driving his enemies to cast a silver bullet in order to murder him.