Felicium was an addictive narcotic, as well as a vaccine, produced on the planet Brekka.

Felicium originated as a vaccine for a plague that existed on Brekka's neighboring planet, Ornara. The vaccine was supplied by the Brekkians in exchange for goods. Even after the plague was over the Ornarans had unknowingly become addicted to the vaccine, the terrible withdrawal symptoms leading them to think they were still affected by the disease, and Brekka continued to produce it for them in a profitable business. As years passed the Brekkians refined the drug to make it more and more potent.

The euphoria seems to be somewhat sedative and pain relieving, similar to planet Earth's opiates. (TNG: "Symbiosis")