In The Return of the Living Dead, the chemical behind the zombie outbreak is called Trioxin.
The film introduces Trioxin through military experiments involving reanimated corpses. A damaged barrel containing the substance is accidentally opened at a medical-supply warehouse, releasing the chemical and setting the story’s outbreak in motion.
The 1985 film, directed by Dan O’Bannon, is a horror comedy that deliberately plays with conventions associated with Romero’s zombie movies. Its undead are able to speak, move quickly and specifically crave brains. Those traits helped distinguish the film’s mythology from the rules of many earlier zombie films.
The title is often confused with Romero’s Living Dead films because of the similar wording. The Return of the Living Dead is a separate production, although O’Bannon had worked on an early screenplay connected to the broader development history of Night of the Living Dead. Trioxin is the film’s fictional chemical, not a real zombie-causing substance.