The QNB-T16 Truth Serum is described as a "top-shelf martini of sodium-pentothal derivatives" that can make people tell the truth about anything, potent enough to easily work on even trained S.H.I.E.L.D. agents. It is believed to be a fake drug with no truth-serum effects.
According to Ward, it does not actually work, but was rather just something made up to manipulate Skye into believing she had control. However, when Skye asked him about this, Coulson neither confirmed nor denied its effectiveness.
The serum works quickly, taking only a few seconds. After an hour of use, the person who was injected with the serum falls asleep.
- Fitz-Simmons Armory link