HPPD (hallucinogen persisting perception disorder) explained: what it is and what is known

Hallucinogen persisting perception disorder, usually shortened to HPPD, is a diagnosis for a small group of people whose perception has been changed for a long time after taking a hallucinogen. Reviews of the research call it rare and poorly understood (Halpern and Pope, 2003; Martinotti et al., 2018). This article sets out what the term means, what the literature does and does not show about how common it is and which substances are involved, and how it differs from the ordinary after-effects of a trip.

We label claims the same way throughout. Established: shown in peer-reviewed work and not seriously disputed. Preliminary: early, small, indirect or based on case reports. Unresolved: the data are missing or point different ways. Nearly everything about HPPD falls in the second and third groups.

What is HPPD?

HPPD is a recognised diagnosis in which perceptual symptoms that appeared during hallucinogen intoxication come back, or continue, after the intoxication is over. A 2017 systematic review quotes the DSM-5 criteria as having three parts: (A) re-experiencing one or more of the perceptual symptoms that were experienced while intoxicated, with examples such as geometric hallucinations, positive afterimages, haloes and micropsia or macropsia (objects looking smaller or larger than they are); (B) clinically significant distress or impairment in social, occupational or other important functioning; and (C) symptoms not due to a medical condition such as brain lesions, infection or visual epilepsy, and not better explained by another disorder such as delirium, dementia or schizophrenia (Orsolini et al., 2017).

The same review traces the label: the DSM-III-R of 1987 listed "Post-hallucinogen Perception Disorder", and DSM-IV-TR recognised "Hallucinogen-Persisting Perception Disorder (Flashbacks)" (Orsolini et al., 2017). The 2003 review by Halpern and Pope made a related point: the older word "flashbacks" has been defined so inconsistently that it has little value, and many earlier studies are hard to interpret for that reason (Halpern and Pope, 2003). Evidence on the definition: established as a diagnostic category, with the limits below. The central point is criterion B. A re-experienced perception is, on its own, not the disorder. Distress or impairment, and no better explanation, are part of the definition.

The symptoms described in the literature are mostly visual. The 2017 review notes that visual symptoms predominate, with depressive symptoms and thought disorders sometimes co-occurring (Orsolini et al., 2017). A 2018 review lists symptoms such as geometric shapes, haloes, trails and afterimages, and describes the visual disturbances as the main symptom group (Martinotti et al., 2018). A separate 2014 review notes that HPPD often occurs alongside anxiety, attention problems and derealization, which makes its causes hard to untangle (Litjens et al., 2014).

What is the difference between HPPD Type 1 and Type 2?

Some authors divide HPPD into two types. The split is used in parts of the literature and is not part of the DSM-5. The 2018 review attributes it to Lerner and colleagues, calls it "hypothetical" in its own discussion, and states that DSM-5 makes no such distinction (Martinotti et al., 2018).

Source for the table: Martinotti et al., 2018. A 2017 review gives a similar picture from another angle: Type 1 as brief re-experiences of altered perception, mood or consciousness that may be pleasurable and that the person usually knows are unreal, and Type 2 as symptoms such as trails, haloes and palinopsia that are often uncontrollable and disturbing, sometimes with depersonalization, derealization, anxiety or depression (Orsolini et al., 2017). Evidence: preliminary. The 2018 review itself says the two types are separated by prognosis rather than clear clinical presentation, so treat the table as a description of how some authors talk about it, not a classification a person can apply to themselves.

How common is HPPD?

Nobody knows precisely. Halpern and Pope reviewed 20 quantitative studies from 1955 to 2001 and concluded that HPPD appears real but uncommon, that many studies lacked the detail needed to tell how many people met the diagnostic criteria, and that current knowledge was very limited (Halpern and Pope, 2003). The 2018 review describes it as rare and says its prevalence is unknown, which is part of why it is often unrecognised (Martinotti et al., 2018).

Figures that do appear depend on how the question is asked. The 2017 review reports that transient flashbacks have been estimated across a very wide range, 5% to 50% of hallucinogen users, but that HPPD is less likely than flashbacks, and it cites one web survey in which 10% of hallucinogen users reported long-term perceptual changes and 1% reported changes that "drove them mad" (Orsolini et al., 2017). A web survey of volunteers is not a population estimate, and the range shows mostly that "flashback" has meant very different things in different studies. Evidence: unresolved. There is no reliable rate per user, per dose or per session, and none for psilocybin specifically.

We do not cite any rate we could not trace to a primary source we read, and this article does not offer a figure for psilocybin or for any single substance.

Which substances have been linked to HPPD?

LSD is the substance most often reported. Halpern and Pope found HPPD most often linked to illicit LSD use (Halpern and Pope, 2003), and the 2018 review's tabulation of cases found LSD and PCP behind most of the reports it analysed (Martinotti et al., 2018). The reviews list many others: psilocybin, mescaline, MDMA, ketamine, cannabis, salvia and others (Orsolini et al., 2017; Martinotti et al., 2018). A 2014 review reported 31 new cases linked to MDMA and concluded that evidence overall remains sparse (Litjens et al., 2014).

Two cautions apply to those lists. First, being named in a list is not a measure of risk: the reviews gather case reports and small studies, and one source's list of "substances implicated" does not say how many users of each were affected, or whether anything else was taken. Second, the 2017 review states that a link between first use, frequency or amount and HPPD "has not been demonstrated" and that onset can follow a single exposure, while the 2018 review notes it is more often diagnosed in people with earlier psychological difficulties or substance misuse (Orsolini et al., 2017; Martinotti et al., 2018). Evidence: preliminary. Anyone looking for a prediction of who is at risk will not find one in this literature.

Why it happens is also open. Litjens and colleagues note that most reported cases involve drugs that act on serotonin 5-HT2A receptors and propose that an imbalance of inhibition and excitation in early visual processing may be involved (Litjens et al., 2014). The 2017 review calls such mechanisms unconfirmed and says the neural basis, risk factors and cause "still largely remain unknown" (Orsolini et al., 2017). Evidence: unresolved. For how psilocybin differs from other substances on the pharmacology side, see our comparison of psilocybin, LSD and DMT.

How is HPPD different from normal after-effects of a trip?

The literature offers a few ways to draw the line, none of them a test you can run yourself.

• Duration and persistence. The diagnosis concerns symptoms that continue or return after the intoxication is over. Halpern and Pope describe HPPD as lasting months or years in the people it applies to (Halpern and Pope, 2003). A short-lived echo of a trip is what the older word flashback covered, and the Type 1 description above is of brief re-experiences (Martinotti et al., 2018).
• Distress or impairment. Under the DSM-5 criteria quoted above, symptoms must cause clinically significant distress or impairment (Orsolini et al., 2017).
• Another explanation. The criteria exclude symptoms due to a medical condition, or better explained by another disorder, which is why a clinician looks for other causes first (Orsolini et al., 2017).
• Insight. The 2018 review discusses insight, the ability to tell a perceptual distortion from a real event, as one way HPPD is distinguished from psychosis (Martinotti et al., 2018). Insight is not a rule that settles the question. Our article on whether magic mushrooms can cause psychosis covers that separate question.

A frightening or overwhelming trip is also a different thing again: it is distress during the drug's action, and it ends with the drug. If you are worried about someone in that situation now, our guide on helping someone having a bad trip and the emergency options above are the place to start. For the general safety picture, see are magic mushrooms safe?.

How strong is the evidence on HPPD?

Weak. The reviews themselves say so, in similar words.

• Case-based. Few double-blind or case-control studies exist; the literature is case reports, case series and small samples, with widely varying methods and diagnostic criteria, and most of it concerns LSD (Orsolini et al., 2017).
• Definitions have shifted. Many older studies predate the current criteria and used "flashbacks" loosely (Halpern and Pope, 2003).
• Review limits. The 2018 review searched one database (PubMed) and English-language work, and included designs from case reports to small uncontrolled studies (Martinotti et al., 2018). The 2014 review calls the evidence sparse (Litjens et al., 2014).
• Other factors. The 2018 review says HPPD is more often diagnosed in people with earlier psychological difficulties or substance misuse, which complicates any reading of cause (Martinotti et al., 2018).

We do not summarise treatments here. The reviews describe treatment evidence as resting on case reports and small uncontrolled studies, which is not a basis for any advice, and decisions about care belong with a clinician who knows the person.

When should someone see a doctor, and where can they get help in Canada?

If perceptual changes after using a hallucinogen persist, are distressing or interfere with daily life, the sensible step is to see a family doctor or other healthcare provider. They can look for physical and mental health causes, which the diagnosis itself requires. The Government of Canada's get-help page lists a family doctor, psychologist, mental health nurse or social worker as people to contact (Government of Canada).

• Immediate danger or urgent medical need: call 9-1-1 (Government of Canada).
• Thoughts of suicide, or worry about someone else: call or text 9-8-8. The 9-8-8 site says it is available around the clock, every day of the year, and to call 9-1-1 if your safety is at risk (988.ca).
• Possible poisoning or a question about an ingestion: the Ontario Poison Centre lists a toll-free line, 1-800-268-9017, open 24 hours a day, every day of the year (Ontario Poison Centre). The page does not say when to use the poison centre rather than 9-1-1, so if someone is unconscious, struggling to breathe or in danger, call 9-1-1.

Our safety page keeps these pointers in one place. For the wider picture of what magic mushrooms are, start with what are magic mushrooms?.

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