One unique characteristic of lysergic acid diethylamide (LSD) and LSD-like substances is the recurrence of some of the symptoms which appeared during the intoxication after the immediate effect of the hallucinogen has worn off. This recurring syndrome, mainly visual, has not been clearly understood, appreciated or distinguished from other clinical entities by clinicians. The terms Flashback and Hallucinogen Persisting Perception Disorder (HPPD) are used interchangeably in the professional literature. Flashback is a usually short-term, non-distressing, spontaneous, recurrent, reversible and benign condition accompanied by a pleasant affect. In contrast, HPPD is a generally long-term, distressing, spontaneous, recurrent, pervasive, either slowly reversible or irreversible, non-benign condition accompanied by an unpleasant dysphoric affect. Flashback and HPPD appear to be part of a vast and broad spectrum of non-psychopathological and psychopathological states reported by hallucinogen users. Pharmacological agents such as clonidine, perphenazine and clonazepan have been shown to ameliorate this syndrome in some of the individuals seeking treatment.
Lerner, A. G., Gelkopf, M., Skladman, I., & Oyffe, I. (2002). Flashback and hallucinogen persisting perception disorder: clinical aspects and pharmacological treatment approach. The Israel journal of psychiatry and related sciences, 39(2), 92.