Jax is a drug that affects the Psy mind. In mild dosages, it heightened your abilities. You do get addicted rather quickly.
There are quite a few Jax junkies around the San Francisco area.
The drug was used for the Arrows. Ming LeBon used it regularly to keep them compliant and at peak performance.
- Nikita was drugged with a variant of the drug in Bonds of Justice. It limited her telepathy so she could not call for help or reach out to her presumably captured daughter.
- Vasic was able to teleport his body mere millimeters to get the drug out of his system.