There's two guys who are responsible for poisoning you: Lothander and Marek. Lothander is the one who is supposed to approach you again after that and he offers you a way out.
Now, have you talked to Lothander and Marek at all? 'Cause if not, you haven't been poisoned yet either.
If you have talked to the two of them (and only then)Lothander talks to you on the map of Baldur's Gate where all the merchants are (it's the map in BG's centre).
If he's not there you can go to the Blushing Mermaid. On the first floor you can find Marek. Bop him on the head 'til he drops dead and fish the antidote from his sorry remains. Give the antidote to all your partymembers and you're saved.
Hope this helps.
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She says: Lou, it's the Beginning of a Great Adventure
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poison runing through my veins 
Linear Mode
