The Joker is commonly associated with this toxin. It is quite possibly his favorite killing technique.
Even in Batman issue #1 the Joker had this strange technique of killing. He is also the only one known to be unaffected by his venom; In Batman #663 it is stated that "being an avid consumer of his products, the Joker's immunity to poisons has been built up over years of dedicated abuse."
On numerous occasions the Joker is able to create a similar acting toxin with ordinary materials found in a common house hold, but the poisons most commonly thought of as components in it's recipe are hydrogen cyanide and strychnine.
Once the Joker was able to concoct the poison using ordinary chemicals from the Arkham Asylum janitor's closet.
In an issue of Detective Comics, the Joker poisoned Gotham's harbors, causing all the fish in the area to develop a deformed smile resembling his own. The toxin is not harmful to humans as he does not intend to poison anyone. Instead he attempts to copyright his "Joker fish", and thus gain a percentage of all fish sold in Gotham.
There are two types of Joker Venom; one is lethal and the other simply causes insanity. However, both types usually cause fits of uncontrollable laughter.
Simply put, it kills with a smile. One theory is that the venom hyper-stimulates the laughter functions of the brain, leaving the victim unable to breathe.
In some cases, it also gives the victim the same facial appearance as the Joker, with white flesh and blood red lips.
Prolonged exposure to the non-fatal forms can cause permanent brain damage. Also, while under its influence the subject will find anything, even the most heinous act or notion, hilarious.
The most common method for the poison's devilry is as an airborne agent, but it is also used in its liquid form which the Joker places in his squirting flower, or on a small spike worn in the palm of his hand - similar to his joy buzzers.
In Tim Burton's 1989 Batman film, the Joker used a version of his toxin called Smylex to taint all the beauty and hygiene products shipped to Gotham.
The Joker venom was also used in 2013's Batman: The Dark Knight Returns.
The Joker has used his toxin many times in 1992's Batman: The animated series, as well as 2004's The Batman, often in the form of a gas.