A truth serium or truth drug was a type of drug that, when given to a humanoid, will cause them to unwillfully tell the truth. Sodium pentathol is an example of a truth serium.
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 serium be used on uncooperative Robert Crater, so as to determine the location of the salt vampire on the surface of M-113. Doctor Leonard McCoy resisted the suggestion of using it, but was in favor of it, in this case, so that they could get the truth from Crater. Crater, nevertheless, escaped 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")