At the end of a long didactic poem, the Persian poet ‘Attar (d. c. 1221) describes his work as 'medicinal opium’, capable of curing his readers’ irreligiosity. This medicinal metaphor - critical to ‘Attar’s authorial persona (his pen-name literally means 'druggist’) and Sufi poetry more broadly - is explored in this book. Austin O'Malley takes the metaphor seriously, uncovering its poetic assumptions and implications for how didactic poetry was read, understood and experienced in the twelfth century. Drawing on a range of historical and literary sources, including some still in manuscript, he argues that didactic poetry was more than a repository of knowledge. Rather, ‘Attar and other didactic poets aimed to affect their readers on an emotive, temperamental and even bodily level, such that the literary encounter becomes a therapeutic regime in the service of spiritual purification and mystical progress.