This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
A truth serum or truth drug was a type of serum or drug that, when given to a humanoid, would compel them to tell the truth during an interrogation. Sodium Pentathol was an example of a manufactured truth serum found on Earth during the 20th century. (DS9: "Little Green Men")
In 2152, a truth serum was administered to Sub-Commander T'Pol while she was undergoing interrogation by a group of Suliban soldiers aboard a Helix operated by the Cabal. She also suffered aftereffects of the drug, such as becoming dazed and shivering. (ENT: "Shockwave, Part II")
In 2153, Doctor Phlox suggested to Captain Jonathan Archer that he could create a truth serum to use on Degra, in order to determine the location of Azati Prime, but added that it could take weeks to synthesize the correct formula. (ENT: "Stratagem")
In 2266, Commander Spock suggested that a truth serum be used on an uncooperative Robert Crater, so as to determine the location of the M-113 creature on the surface of M-113. The M-113 creature in disguise as Doctor Leonard McCoy resisted the suggestion of using it, but was in favor of it, in this case, so as not to raise suspicion. Crater, unfortunately, was killed by the M-113 creature prior to the administration of the drug. (TOS: "The Man Trap")
When Thelev was discovered to be an Orion spy serving in the Andorian delegation aboard the USS Enterprise in 2268, he was subjected to questioning under a verifier scan and given a truth drug. Unfortunately these tactics did not reveal anything, which suggested to Spock that his mind had been conditioned against such means as part of a preconceived plan. (TOS: "Journey to Babel")
In 3189, Aurellio prepared a truth serum so as to compel Cleveland Booker to reveal how to reach Theta Zeta. However, the serum proved ineffective, which Booker claimed was due to his being a former courier and thus well-acquainted with microdosing. (DIS: "That Hope Is You, Part 2")