From what I understand, Truth Serums are hypnogogic in nature, making the subjects highly suggestable, in so far as to induce false memories from the statements that the interegators make.

So, if you were under the influence of truth serum, and someone asked you what you were doing in the middle of the desert at a secret UFO base, you would fantasize a situation where you had been at Area 51, trying to break out a bunch of hairless alpha-omegan BEMs with the help of the Lone GunMen, while Muldeur and Skully were keeping a lookout just beyond the fence.

You'd be convincing enough, and just might come out from under the serum beleiving everything you imagined.

So, Truth Serum is not all its cracked up to be.