As originally published, De Quincey's account was organized into two parts:
- Part I begins with a notice "To the Reader," to establish the narrative frame: "I here present you, courteous reader, with the record of a remarkable period in my life...." It is followed by the substance of Part I,
- Preliminary Confessions, devoted to the author's childhood and youth, and concentrated upon the emotional and psychological factors that underlay the later opium experiences — especially the period in his late teens that de Quincey spent as a homeless runaway in Oxford Street in London in 1802 and 1803.
- Part II is split into several sections:
- A relatively brief introduction and connecting passage, followed by
- The Pleasures of Opium, which discusses the early and largely positive phase of the author's experience with the drug, from 1804 until 1812;
- Introduction to the Pains of Opium, which delivers a second installment of autobiography, taking De Quincey from youth to maturity; and
- The Pains of Opium, which recounts the extreme of the author's opium experience (up to that time), with insomnia, nightmares, frightening visions, and difficult physical symptoms.
- Another "Notice to the Reader" attempts to clarify the chronology of the whole.
Though De Quincey was later criticized for giving too much attention to the pleasure of opium and not enough to the harsh negatives of addiction, The Pains of Opium is in fact significantly longer than The Pleasures. However, even when trying to convey darker truths, De Quincey's language can seem seduced by the compelling nature of the opium experience:
- "The sense of space, and in the end, the sense of time, were both powerfully affected. Buildings, landscapes, &c. were exhibited in proportions so vast as the bodily eye is not fitted to conceive. Space swelled, and was amplified to an extent of unutterable infinity. This, however, did not disturb me so much as the vast expansion of time; I sometimes seemed to have lived for 70 or 100 years in one night; nay, sometimes had feelings representative of a millennium passed in that time, or, however, of a duration far beyond the limits of any human experience."