The Bliss is an expensive drug that only the rich can afford. It is a treatment (not a cure) for the Flare that delays the inevitable by slowing down brain function, as the disease takes over more quickly if the person's brain is working harder.
While it slows the progress of the Flare, it also makes the user more relaxed and "zoned-out." Users will often simply stare blankly at the sky, relax, etc., which makes it easy to tell if a person is on the drug.
In Denver, Thomas sees someone arrested for having the Flare and not telling anyone. The man was given away by the fact that he was on the Bliss.
Trivia
- Brenda mentions that the Cranks living in the Scorch are not given any Bliss, this implies that the Cranks were majorly used as part of the trials.