Smallville: Season 4 Episode 10

Scare

A Luthorcorp experiment is released into the atmosphere – a toxin that causes its victims to hallucinate their worst fear. Chloe, Clark, Lana, and Jason all fall fictim to the toxin, and Lex, desperate to find a cure, takes a dangerous antidote. Meanwhile, Lionel is released from prison and his conviction overturned thanks to a mysterious benefactor.