Perhaps more indicative of the insanity of the Joker than anything else about him, Smilex is a deadly poison that causes horrific facial deformations, usually in form of a rictus grin.
Smilex is made in both liquid and gaseous forms – the gas resembles a green smoke. The liquid can be mixed into almost any foodstuffs.
The puzzling thing about Smilex is the business plan behind its distribution. The Joker gives it away, seeking the widest possible range of people in Gotham, with the intent of killing them. Surely, this is insanity by any modern standard: the total absence of anything even resembling a profit motive – in fact, the Joker probably took a loss on the deal.