A mind-altering drug that makes you incapable of lying used by the CIA, police, and Nazi interrogators during the 20s through 40s.
They slow the speed at which your body sends messages from your spinal chord to your brain, inhibiting your ability to perform high-functioning tasks like walking a straight line or lying. Research shows you are more likely to tell the truth under the effects of a truth serum but also more likely to make complacent or outright false statements. It also instills a warm, friendly feeling towards your interrogator, prompting you to tell him what he wants to hear, rather than what's true. They lead to too many useless statements, so it's difficult to sift out the truth.
Truth serums are illegal for use in interrogation. They were ruled admissible in US courts in 1963 as statements were "unconstitutionally coerced" and in violation of the fifth amendment.
(https://www.businessinsider.com/is-there-such-a-thing-as-truth-serum-2014-10)