Occasionally, a person will find that some perceptual aspect of hallucinogen drug intoxication—typically, experiencing visual “trailers” or a static-like overlay (“visual snow”) of vision, or the persistence of after-images as the visual field moves—continues past the end of the hallucinogen experience or reemerges in the days, weeks, or even longer after the experience. Altered perceptions, mood, and/or consciousness as previously experienced during the intoxication will involuntarily activate and, as such, can trigger tremendous concern and anxiety in those experiencing such phenomena. When these symptoms meaningfully affect the psychological competence or clinical functioning of the individual, he or she may be diagnosed with hallucinogen persisting perception disorder (HPPD).

There is some controversy over HPPD because several nonhallucinogenic substances have also been described as inducing similar phenomena, including ...