A truth serum made one incapable of lying.
Yvonne Hartman used a truth serum on Matthew to determine his trustworthiness after she surmised that there was a mole in Torchwood One. However, he was immune to the effects due to Excellium, meaning that he was able to convince her that he was not the one to supply the drug to Blind Summit. (AUDIO: Blind Summit)
Jack Harkness told Orr to use truth serum on Yvonne Hartman. However, she elected to use it on Xander Vaughn instead, resulting in Jack's connections to Red Doors being made known. (AUDIO: Poker Face)