Spectrox

Spectrox was a substance harvested on the planet Androzani Minor.

It was naturally found (in dense, white, fibrous clusters) in the chrysalis of the Spectrox Bat, a species that lived in the many caves that riddled the dry desert planet.

Unrefined spectrox was a powerful neurotoxin, capable of killing even a Time Lord. Refined spectrox, taken in regular doses, could prolong a human lifespan to about twice its ordinary length, greatly slowing the aging process.

Because it was so dangerous in its raw form, it was harvested by androids and brought to special refineries. After this, the spectrox was shipped to Androzani Major and then the rest of the galaxy.

Refined spectrox needed to be taken daily, though aging would only continue after a week or so (of not taking it). Because of its uses and rarity, spectrox was one of the most valued substances in the galaxy and many battles (both covert and overt) were fought over its possession and distribution. (DW: The Caves of Androzani)