Salt

Table salt, also known as sodium chloride is a condiment often used by Humans.

Due to its unique chemical properties, disrupting the osmotic balance and eletrical conductivity of their cells, table salt could act as a defense against the Fendahl. The Doctor could kill Fendahleen by firing table salt out of a rifle. The power of salt to ward off evil, then, entered into Human folklore in the form of throwing salt over one's shoulder for good luck. (DW: Image of the Fendahl)

Minor reference
Adric managed to irk Tegan by requesting that she pass her "the sodium chloride". At first, she did not know what he meant. (DW: Four to Doomsday)