F16 — Mental and behavioural disorders due to use of hallucinogens

What is this category?

F16's highest-yield teaching points are acute intoxication (perceptual alteration is the primary, expected effect rather than an incidental one) and the residual/late-onset subtype, which captures a distinctive phenomenon — brief, spontaneous recurrences of perceptual disturbance well after the drug has been eliminated, popularly known as "flashbacks."

Core diagnostic concept

Unlike most other categories in this block, marked perceptual alteration during hallucinogen intoxication is the expected primary effect of the substance, not itself evidence of a psychotic disorder. The classification question is therefore less "is perception altered?" (expected) and more "does the disturbance persist well beyond the expected intoxication period, recur without further use, or amount to a sustained delusional or hallucinatory picture beyond perceptual alteration?"

Key diagnostic features

Acute intoxication phenomena

Marked alterations of perception (visual distortions, synaesthesia), mood, and thought during use — the substance's primary, expected effect, resolving as it clears.

Psychotic disorder

A more sustained delusional or hallucinatory picture going beyond expected perceptual alteration, persisting beyond the intoxication period.

Residual and late-onset psychotic disorder (flashbacks)

Brief, spontaneous recurrences of perceptual disturbance experienced during past intoxication, occurring well after the drug has been eliminated and without further use — a phenomenon relatively distinctive to this category.

Dependence syndrome

The six-criteria cluster applies here too, though classic hallucinogens are less consistently associated with a clear physiological withdrawal state than other substances in this block.

Temporal relationship to use

As with F12 and F14, timing relative to use — and, distinctively here, whether disturbance recurs without any further use at all — is the decisive detail.

How to weigh evidence

These learning labels are teaching aids, not official ICD-10 terminology (see Sources and Method).

• supports Marked visual distortions and altered sense of time during hallucinogen use, resolving fully within several hours as expected.
• consistent not decisive The person has used a hallucinogen once. A single use with an expected, fully-resolving intoxication does not indicate a disorder requiring further classification.
• raises alternative Brief, unprompted recurrences of the same visual distortions weeks after last use, with no further hallucinogen use in the interim.
• contradicts or weakens No further perceptual disturbance at all once the initial intoxication period passed.
• missing and necessary to ask Whether any perceptual disturbance has recurred since the intoxication period ended, and if so, whether further use has occurred in between.

Duration and course

Minimum duration: Dependence syndrome, where present, follows the same three-or-more-criteria pattern as elsewhere in this block, though this is less commonly established for hallucinogens than for most other substances here.

Course: Intoxication typically resolves within hours; flashback-type recurrences are brief (seconds to a few minutes) and can occur sporadically over an extended period after use has stopped entirely.

Included presentations

• Transient intoxication with marked perceptual, mood and thought alteration, resolving as expected.
• A more sustained delusional or hallucinatory picture temporally linked to hallucinogen use, beyond expected intoxication.
• Brief, spontaneous recurrences of intoxication-type perceptual disturbance occurring without further use.

Exclusions and distinguishing presentations

Four-character subtypes (orientation)

A learner should not classify every altered perception during hallucinogen use as a psychotic disorder — marked perceptual alteration is this category's expected primary effect. Subtypes beyond F16.0 and F16.7 are taught here for orientation only and are not separately assessed in this pilot.

F16.0 — Acute intoxication

Marked perceptual, mood and thought alteration during use — the substance's expected primary effect. (in_depth)

F16.1 — Harmful use

A pattern causing demonstrable harm without the fuller dependence cluster. (taught for orientation only, not separately assessed in this pilot)

F16.2 — Dependence syndrome

Three or more of the six dependence criteria present together — less commonly established for classic hallucinogens. (taught for orientation only, not separately assessed in this pilot)

F16.3 — Withdrawal state

Not consistently well-characterised for classic hallucinogens. (taught for orientation only, not separately assessed in this pilot)

F16.4 — Withdrawal state with delirium

Rare with this category's substances. (taught for orientation only, not separately assessed in this pilot)

F16.5 — Psychotic disorder

A more sustained delusional or hallucinatory picture temporally linked to use, beyond expected intoxication. (taught for orientation only, not separately assessed in this pilot)

F16.6 — Amnesic syndrome

Not a classically described feature of this category. (taught for orientation only, not separately assessed in this pilot)

F16.7 — Residual and late-onset psychotic disorder

Brief, spontaneous recurrences of intoxication-type perceptual disturbance occurring without further use ("flashbacks"). (in_depth)

F16.8 — Other mental and behavioural disorders

A hallucinogen-related presentation not fitting the other listed states. (taught for orientation only, not separately assessed in this pilot)

F16.9 — Unspecified mental and behavioural disorder

A hallucinogen-related disturbance identified but without enough information to classify further. (taught for orientation only, not separately assessed in this pilot)

External causes to check first

Categories in this block require that the disturbance is not better explained by another external cause listed below. These short capsules are reminders, not full teaching modules.

Worked example — expected intoxication versus a residual and late-onset psychotic disorder

A 24-year-old used a hallucinogen once at a festival several months ago, experiencing visual distortions and altered time perception for a few hours that resolved as expected by the next morning. Since then, he has had two brief episodes — each lasting under a minute — of similar visual distortion appearing without warning while otherwise sober, with no further hallucinogen use in the interim. He finds these episodes unsettling but otherwise functions normally.

1. Step 1 — Check the original intoxication: the initial episode's perceptual alteration during use, resolving fully by the next morning, is a straightforward, expected acute intoxication (F16.0).
2. Step 2 — Check for further use: he confirms no further hallucinogen use since that single occasion.
3. Step 3 — Characterise the later episodes: brief, spontaneous recurrences of the same type of perceptual disturbance, occurring in the clear absence of further use.
4. Step 4 — Weigh against ongoing intoxication or a sustained psychotic disorder: because there has been no further use and the episodes are brief and self-limited rather than a sustained delusional or hallucinatory picture, this does not fit F16.5.
5. Conclusion — The later episodes are best classified as a residual and late-onset psychotic disorder (F16.7) — the classic "flashback" phenomenon — distinct from both the original intoxication and from an ongoing substance-induced psychotic disorder.

Interactive practice

• Type A The colours were part of the plan Recognition / application · Foundation
• Type B It came back, out of nowhere Boundary / differential · Boundary

Retrieval prompts

Try answering each of these from memory before revealing where to check. Nothing you think or type here is sent or saved anywhere.

1. Without looking back at the page, state why marked perceptual alteration during hallucinogen intoxication is not automatically evidence of a psychotic disorder.

   Reveal answer location

   Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.

   I recalled this I need to review this

2. Name the feature that distinguishes a "flashback" (F16.7) from ongoing intoxication.

   Reveal answer location

   Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.

   I recalled this I need to review this

3. Explain in one sentence why the presence or absence of further use matters for classifying a recurrence.

   Reveal answer location

   Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.

   I recalled this I need to review this

These self-checks are a personal study aid only, visible to you on this page for this visit — they are not stored and do not affect your mastery dimensions.

Related glossary terms

• Flashback

  The common informal name for a residual and late-onset psychotic disorder following hallucinogen use: a brief, spontaneous recurrence of perceptual disturbances experienced during earlier intoxication, occurring well after use and without any further use.

  Open full glossary entry →