From Gineipaedia, the Legend of Galactic Heroes wiki
Thyoxin (Japanese: サイオキシン麻薬) is a highly addictive synthetic psychoactive drug that was notoriously used by the Terraist Church to brainwash and control its followers.
The identity of the creator of thyoxin as well as its exact date of creation are both unknown. However, it was already known in the Galactic Empire by the 780s UC, where it became prevalent as a recreational drug in certain sectors of the Imperial military as well as some rural areas within the Empire. (DIS: 'Chapter I')
Although thyoxin abuse was a serious issue within the Imperial military, its adverse impact on the battle performance of Imperial soldiers was only documented once during the Battle of Arlesheim in 792 UC (483 IC / 3592 CE), where rampant thyoxin abuse within the Keyserling Fleet caused its soldiers, in a wave of drug-induced hallucinations, to attack prematurely and expose the fleet's position to the Alliance forces it was supposed to ambush. The result was a fiasco for the Empire and the battle was bitterly remembered as a dishonourable defeat. (DIS: 'Chapter I', 'Chapter IV')
Besides its use as a recreational drug, thyoxin is also rumoured to be secretly employed by both the Empire and the Free Planets Alliance to brainwash people. Its most notorious user however was the Terraist Church, who had no qualms in resorting to spiking the food and beverages of its believers with thyoxin so as to turn them into an army of mindless fanatics for the Terraist cause. (LOGH: 'Holy Land')
The only known form of thyoxin is that of a white powder which is soluble in water. It also appears to be imperceptible in minute quantities. When ingested, it imparts to its user an incredible sense of euphoria. It is however an extremely potent hallucinogen and teratogen which ultimately harms not only its user but also his or her offspring in the form of severe birth defects. (LOGH: 'Holy Land', DIS: 'Chapter I')