Smylex was Joker's codename for any of his poisoned products that, when paired in specific combination, would cause their victims to begin laughing uncontrollably, and then fall down, with their facial muscles pulled taut in a rictus grin. It could also be processed in a gaseous state.
Batman[]
The Joker uses "Smylex" specifically in gas form, in an attempt to kill everyone at a celebration in the center of Gotham. He also contaminates household products with binary compounds of the chemical, which is only lethal when mixed together the right way.