A drug refined on the planet Brekka and sold to neighbouring Ornara as the treatment for a plague, when in truth the plague is long cured and the Ornarans are addicts.
Felicium is the subject of the episode Symbiosis. The Enterprise-D rescues survivors and cargo from a failing Ornaran freighter, and finds the passengers far more concerned about the barrels than about their dead crewmates.
Two hundred years earlier felicium did cure a plague on Ornara. The Brekkians have gone on supplying it ever since, and their whole economy consists of making it, while Ornara provides everything else they need. Doctor Crusher works out that the Ornarans' symptoms are simply withdrawal.
The Brekkians know it. Picard holds that the Prime Directive forbids him from telling the Ornarans, but it also lets him refuse the parts their decaying freighters need, so the supply will soon end by itself.