Travel to Tarren Mill in Hillsbrad Foothills and deliver the Sample of Zanzil's Mixture to Serge Hinott. To get to Tarren Mill, take the Zeppelin to the Undercity and follow the road south through Silverpine and towards Hillsbrad. Follow the signs!
Take a sample of the mixture to Serge Hinott in Tarren Mill.
If anyone can cure you, it's Serge. Live long enough and Serge will show you the ways of poison creation and use.