Tantalum bi-lithium thalo-dihydroxide, also known as TLTDH gas, was a chemical that became a colorless, odorless, non-flammable gas at 1 ATM and temperatures within Human norm. Composed of molecules of tantalum, lithium, hydrogen and oxygen, TLTDH was an early anaesthetic amongst the Vulcan and Romulan civilizations. A noted effect was the production of laughter amongst them after exposure to the gas as well as feelings of exhilaration, euphoria and sometimes unconsciousness, somehow the way nitrous oxide affects humans.

In the post-industrial but pre-spaceflight era, a Vulcan social problem emerged when crude TLTDH became popular to "cook up" from non-conductive tantalo-lithial compounds that were commonly used as electrical insulation.

In the 2260s, it was synthesized by the crew of the USS Enterprise in order to cure the accidental infection of Spock and Romulans at the ARK-7 facility by the Oroborus virus. The effects of the gas knocked out the Romulan invaders that had boarded the starbase. (TOS video game: 25th Anniversary)