Pooka Pus is an ingredient used by elves in medical mixtures. In Flashback, Elwin offers to let Ro help him make a serum containing Pooka Pus in which Sophie must soak her arm to help with her echo. Ro responds positively to this offer and Sophie claims that she is starting to hate elvin medicine almost as much as human medicine, suggesting that Pooka Pus is not a pleasant substance.
The liquid form of pooka pus is a powerful cell regenerator, and relatively easy to find. The solid form is incredibly rare, and can tackle the strongest infections.