Antidote X, as it's name implies, is an antidote to Chemical X. What's known about it is that it's a thick, red liquid (though it's black in the movie) that takes away powers given by Chemical X. It first appears in Mo Job. It is implied by the Professor that it is cherry-flavored. It was first created in the The Powerpuff Girls movie by Professor Utonium by means of taking away Mojo Jojo's powers which he had obtained from additional Chemical X making him into a giant monstrous version of himself.