AKA Rapture, "The Big R"
Legal narcotic produced by Sweet Dream Pharmaceuticals. Usually taken in pill form, but when April is discussing the recent strange events in Venice with Charlie, she mentions that "rapture gas" might be responsible, implying it is capable of producing collective hallucinations. When she sees a musician playing badly outside the Roma Gallery, April comments that she can tell he's high on Amathin because the sense of rhythm is the first thing to go.
While at colonial registration, April notices an advertisement for 'Rapture Revisited': "They're relaunching the original Amathin drug, uh, sorry -- stimulant... Twice as effective for half the cost. Time to usher in a whole new generation of drug addicts." Colonial representatives seem to be in league with Sweet Dream Pharmaceuticals: the latter get people addicted to Rapture, making them susceptible to the indentured labour contracts offered by the colonies. This is what happened to Warren Hughes' family, after his father Ellis Hughes signed them all up in exchange for a life-time's supply of Amathin.
The manual for The Longest Journey mentions "designer drugs that make dreams real", which may be referring to Amathin.
The names of Amathin sometimes differ heavily in translations of the game. While e.g. in the german version of The Longest Journey, the scientific name stays the same, the drug is otherwise referenced as "Exta-cool".