Spectrox was a substance harvested on the planet Androzani Minor.
It was naturally found (in dense, white, fibrous clusters) in the chrysalis of the queen bat, a species that lived in the many caves that riddled the dry desert planet.
Unrefined spectrox was a powerful neurotoxin that caused spectrox toxaemia, capable of killing even a Time Lord. The only known cure was the milk from the queen bat. Refined spectrox, taken in regular doses, could prolong a human lifespan to about twice its ordinary length, greatly slowing the ageing process.
Refined spectrox needed to be taken daily or ageing would resume 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. (TV: The Caves of Androzani)