A truth serum or truth drug was a type of drug that, when given to a humanoid, will cause them to unwillfully tell the truth during an interrogation. Sodium Pentathol was an example of a manufactured truth serum found on Earth. (DS9: "Little Green Men")
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 uncooperative Robert Crater, so as to determine the location of the salt vampire on the surface of M-113. The salt vampire 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 salt vampire 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 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 2369, Captain Jean-Luc Picard was drugged and interrogated by the Cardassian officer Gul Madred. (TNG: "Chain of Command, Part II")
Chakotay is given a Kazon truth drug
Commander Chakotay was beaten and questioned with little success under some kind of truth drug by the Kazon First Maje Culluh in 2372. (VOY: "Maneuvers")