Z-67

Z-67- also referred to by the Twelfth Doctor as 'the Imbecile's Gas'- was a nerve gas that unravelled the DNA of Zygons. Naval surgeon Sullivan working for UNIT developed it after the attempted Zygon invasion of the 1970s or 80s, working on captured Zygons at Porton Down. The Doctor took it from UNIT, as well as the formula; he pretended it could be released by the Osgood Box. (TV: The Zygon Invasion/The Zygon Inversion) Z-67