The antidote is the formula that is needed to save mankind in The Tribe. It is made up of two common plants and another secret ingredient. It's discovered by Lex, Ebony & Tai-San at the start of series 2 in one of the government buildings. Tai-San finds the formula in a separate room, memorizes it and then burns it so other people cannot misuse it to gain power and wealth. In Series 2 Tai-San lets Bray also know the secret ingredient in case anything may ever happen to her. The antidote itself is a yellowish wee colored formula that has a nasty taste.
The antidote is, ironically, partially responsible for the death of Bob in Series 2. When Ebony secretly mixes some poison with some of the antidote in a container meant for Tai-San, it is accidentally knocked over and spilt on the floor, which is lapped up by Bob.