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 electrical conductivity of their cells, table salt could act as a defense against the Fendahl. The Fourth 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 Hervoken were also similarly vulnerable to salt. (NSA: Forever Autumn)

After rejecting pure salt for being "too salty," the Tenth Doctor ate anchovies containing salt in order to eject the cyanide from his body. (DW: The Unicorn and the Wasp)

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