@(๑˃ᴗ˂)ﻭ Niko No, the poison doesn't come from the contract, the poison is from the ML. Not to spoil too much, but one main reason why everyone tries to stay away from the ML is because one of his passive abilities is basically poison, an extremely deadly poison, in his blood, and other bodily fluids. Just touching him can lead to a severe poisoning. It's also why the gas mask is essentially his signature look. At this point in the story, MC is not aware of the poison before this incident. The antidote was used because ML expected it to neutralize his poison, but as it was clearly demonstrated in this chapter, the antidote did not work, and if MC did not have the S+ rank poison recovery ability, he would have died. The novel details it a bit better than this webcomic does, but mostly because you can include more details in text form than you can in a few drawn panels.