Among the complications associated with the use of classic psychedelics, flashbacks, hallucinogen persisting perception disorder (HPPD) and reactivations are most specifically linked to their use. These three phenomena share the occurrence of perceptual disturbances resembling those experienced under the acute effects of psychedelics, but HPPD differs from flashbacks and reactivations due to its persistent nature.Existing evidence suggests that HPPD has a generally low relative prevalence, though in some cases, it may reach considerable clinical relevance. In contrast, flashbacks and reactivations may be more common but are less frequently documented, as they typically do not result in significant distress or treatment-seeking behavior.Many patients presenting with post-psychedelic complications assume they have HPPD; however, only a minority actually meet diagnostic criteria, with others suffering from different psychedelic-related complications. This complicates epidemiological estimates of HPPD and underscores the importance of comprehensive differential diagnostic assessment. Subsuming non-pathological phenomena like flashbacks to HPPD inappropriately inflates prevalence estimates.In very rare cases, HPPD may develop into a chronic condition requiring long-term pharmacological treatment, while in most cases, HPPD spontaneously subsides within one year or diminishes to a tolerable level. It is therefore essential to avoid emphasizing negative prognoses, as they may influence treatment outcomes.This chapter provides an overview of the phenomenology, epidemiology, diagnostic classification, and differential diagnoses of flashbacks, HPPD, and reactivations. As evidence-based treatment options are not yet available, current knowledge is derived from case reports and clinical experience.
