Testing the Tonic is a Horde quest in Thousand Needles that requires players to take the Crate of Reagents from Wizlo Bearingshiner in Whitereach Post to Magistrix Elosai in Freewind Post so she can finish her experiments in finding a cure for the Blood Elves' magical addiction. You must have completed  in order to get this quest.
This quest was introduced in patch 2.0.1.
<Wizlo retrieves the water sample from the robot and places it in a crate with the boulderkin earth and several other items.>
There you go, that's everything on Magistrix Elosai's list. Tell her that it's been a pleasure working with her and that I wish her luck in her research.
Were you and Wizlo able to gather the reagents?
Excellent, it appears that everything is here. Now, all I have to do is mix everything together and give it a try. If this tonic does what my research indicates it should do, I may have discovered a way to lessen the effects of my people's magic addiction.
You're about to witness history in the making, <name>. Just think, without your help, this wouldn't have been possible.
After the quest is completed, the following occurs:
- <Magistrix Elosai removes the reagents from the crate and mixes portions of each in a flask.>
- Magistrix Elosai says: I've endured much hardship to complete my research and with your help, <name>, I've finally done it. Here's to a brighter future!
- Magistrix Elosai says: I feel... lightheaded... What's... happening to me?
- Magistrix Elosai says: No, this can't be! This tonic... it was supposed to reverse my addiction, not hasten my descent into the ranks of the Wretched!
- Magistrix Elosai says: All those years of research... and for nothing! Perhaps they were right, perhaps this is something we must learn to live with.