Scarecrow often uses fear toxin to strike fear into his victims. They are able to cause hallucinogens,and paranoia; some versions of the toxin make people unafraid of anything making them do risky tasks,where the end result could be death.Scarecrow's toxin can show peoples deepest fears specifically or heighten other fears they didn't know they had. Joker remains immune to his gas ,but it can even effect Batman someone who has mastered fear ,although Batman has been able to fight it. Crane's gas can scare victims to death or hypnotizing them bending them to his will. He has used the gas as a pheromone, once placing it on Batman to make his own friends fear him without reason. Scarecrow used it to make him appear more difficult in combat making some think his strength is beyond human making it affect them as if it was.
Fear Toxin/ Venom- When Bane employed Scarecrow to combine his Fear Toxin with Bane's Venom it has a strange effect. The infected person gained great strength and complete fearlessness. The first versions of the formula has the side effect, that when it wore off the subject would bleed out his eyes and be in great pain. This was fixed after many tests on the inmates of Arkham Asylum and when Bane took the formula himself. Originally in The Dark Knight: Knight Terrors.
Super Toxin 451-A- A formula original designed by Dr. Crane, the father of Scarecrow. 451-A is created from the usual Fear Toxin ingredients but has the added ingredient of sweet from children under the effects of Fear Toxin. This makes it more potent. Originally in The Dark Knight #13.
In the film, the fear toxin is being dealt out by the Mob (namely Falcone) throughout the city and to Dr. Crane, which he uses in his experiments (using his mask to enhance the effects). Crane is working with the League of Shadows to bring down Gotham by using the compounds of a rare blue flower that causes hallucinations in a more powerful form.The Toxin is only affective when it is in a vapor form, so a microwave emitter (which evaporates water) is stolen from Bruce's company. Crane and his workers have been pouring the toxin in liquid form into the main waterlines. When Ra's Al Ghul and his League of Shadows arrive to Gotham, it is revealed that the microwave emitter will be activated, evaporating the water and activating the toxin, creating chaos and panic throughout the city. Batman (with the help of Gordon) manages to stop Ra's' plan. Later Crane starts a drug business trafficking the toxin but stopped by Batman.
One year later Scarecrow is interrupted by Batman wannabes while dealing it to some mobs. He's later stopped by the real one.
Batman: The Animated Series
We first see fear toxin being used in "Nothing to Fear", the third episode of the first season. Scarecrow sprays a security guard with the chemical while breaking into a university's vault which causes him to hallucinate that he is covered in spiders. Scarecrow explains the fear toxin causes the victim to see their worst fear which, in the guards case, is arachnophobia. A short time later, Batman is hit with a dart tipped with fear toxin which causes him to have a number of hallucinations. He later recovers from these effects.