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)

The Tenth Doctor ate food containing salt in order to eject the Cyanide from his body. (DW: The Unicorn and the Wasp)

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)