The QNB-T16 Truth Serum is the "top-shelf martini of sodium-pentothal derivatives", according to Agent Coulson in the "Pilot", which can make people tell the truth about anything. It is potent enough to easily work on even top S.H.I.E.L.D. agents.
So far, the only person it is known to have been used on is Agent Ward to help Skye trust S.H.I.E.L.D. so she could join Phil Coulson's team. In "The Asset" Ward informs Skye it was all a ruse, Coulson gave him a placebo, and S.H.I.E.L.D. has no such compound. Skye attempts to confront Coulson on this point, but gets no direct answer.
The serum works quickly, taking a few seconds to work. After an hour of use, the person who was injected with the serum falls asleep and returns back to normal when awakened.
- The 'martini' description is presumably a James Bond reference.