Kodoku (roughly translating to loneliness) is a recurring drug that is introduced in Fushigi Yuugi, and has been used simultaneously in many installations. It is a special, powerful poison that alters your state of mind. It has been noted to posess brainwashing capabilities, and the power to erase memories while under its effects.
It was first seen when it was slipped to Tamahome by Nakago and Yui in Kutou, turning him against Miaka and the rest of the Suzaku Seven. It caused him to briefly forget his alliegance to Miaka and attack her, causing serious injury to her arm, as well as nearly killing Tasuki. He is able to overcome it, however, with the power of Miaka's love for him.
The poison re-appeared in the first OVA when Kaen slips it to an unsuspecting Tamahome, poisoning him for the second time. However, this time, he much more easily breaks free of the poison's influence.