Silver bullet

In human mythology, bullets made of silver could be used to kill a werewolf. The Tenth Doctor, when faced with a "real" werewolf, the Lupine Wavelength Haemovariform, sarcastically asked Rose Tyler if she had any silver bullets on her before running. The werewolf itself was unharmed by ordinary bullets; as the Doctor said to Captain Reynolds, "Bullets can't stop it." (TV: Tooth and Claw)

Captain Cook claimed that Mags, a Vulpanan werewolf, would have died at the hands of a silver bullet back on Vulpana if it was not for him. (TV: The Greatest Show in the Galaxy)

When the Brigadier listed UNIT's inventory of bullets to the Seventh Doctor, the Doctor asked if they had any silver bullets, adding "you never know". The Brigadier promptly asked the Quartermaster Sergeant for silver bullets. He later utilised silver bullets to defeat the Destroyer. (TV: Battlefield)