Have the rest of the antidote but what do I do now?

Let's see if I can get it right this time.

The end of the poison quest is confusing - because nothing really happens to show it's finished correctly. I think Six's website explains it clearly (Chapter 13), but one thing for sure is that the antitode is not actually what cures you. The act of killing Marek completes the quest and the dialog sceen shows XP gained and Poison cured, so if you can then scroll back the dialog to check it. If you search the forum I'm sure someone has explained it better.

Thanks Midget. Naa it is too late to scroll back. Nevermind.

Thats's a pretty odd ending. Well it is done and that's the main point.