Lethium is a drug used to induce sleep. In The King of Attolia, it is referred to as "poppy juice", which has a similar effect in the real world. Eugenides is frequently forced to use the drug, although he hates it, at one point saying that it "makes him feel as though [he's] been dead". It is unlikely that Lethium is addictive, as Eugenides has no trouble with giving up the drug after using it for months on end in The Queen of Attolia to drive away his Nightmares.