In my opinion, this is the absolute worst unlock in Hitman. All it does is make things worse for you.
What does it do? Basically, you can "poison" the target's food or drink with this vial. When they consume the food or drink, they get knocked out...They fall to the floor, unconscious. But, here's the problem. They will consume their food in a public area, people will freak out, security will storm and huddle over the knocked out body, and will eventually bring them back to life. Oh, and everyone will then be on high alert!
I absolutely see no redeeming qualities in this piece of junk! All it does is makes things much worse for you, and fails to do the most important job, like oh, I don't know...kill the target!?