Salt

Salt also known as sodium chloride (DW: Four to Doomsday) was a mineral that was used on Earth as a preservative and flavour enhancer. Around 6100 BC Osirians utilised salt-based spacecraft. (BFA: The Bride of Peladon)

Clyde Langer suggested that the salt in his cold chip sandwich was what the Slitheen were allergic to. Luke Smith noted that if the Slitheen were made mostly of water it would dehydrate them like slugs. However, that theory debunked as salt was contained in the bread and water of the sandwich another Slitheen ate. It was soon realised that the Slitheen were calcium-based and allergic to vinegar. (SJA: Revenge of the Slitheen)