From Heroes Wiki
Opium is a narcotic that can be found in the seed pods of opium poppies. It was used on Hiro Nakamura by Adam Monroe to prevent him from using his powers.
Kensei ties up Hiro and places a dollop of opium beneath his nose. This causes Hiro to be drowsy and somewhat incapacitated, so he can't teleport away from Whitebeard's camp. Later, Yaeko removes the opium and a sluggish Hiro is able to use his powers.
"Why is everything so purple?"
- - Hiro (to Yaeko) (Out of Time)