Category · F40-F48 block
F42 — Obsessive-compulsive disorder
Terminology note: Obsessions are unwanted, intrusive thoughts, images, or urges that the person recognises as their own but experiences as senseless or excessive (ego-dystonic) and tries to resist. Compulsions are repetitive behaviours or mental acts performed to reduce the anxiety obsessions cause. Preserved insight — the person knows the thoughts are irrational or excessive, even if they cannot stop them — is what distinguishes OCD from a primary psychotic disorder.
What is this category?
F42 covers presentations dominated by obsessions, compulsions, or both. F42.0 covers predominantly obsessional thoughts or ruminations without prominent compulsive acts; F42.1 covers predominantly compulsive acts (obsessional rituals) without prominent obsessional thoughts; F42.2 covers a genuine mixture of both, which is the most common presentation in practice.
Core diagnostic concept
The defining feature of F42 is the obsession-compulsion cycle together with preserved insight: an unwanted, intrusive, ego-dystonic thought generates anxiety, and a compulsive ritual is performed to reduce that anxiety, providing only temporary relief before the cycle repeats. The person recognises the thoughts as excessive or irrational, which is what distinguishes OCD from a delusion or another primary psychotic symptom.
Key diagnostic features
- Obsessions are ego-dystonic
- The person experiences the thoughts as unwanted, intrusive, and inconsistent with their own values or intentions, not as true beliefs they endorse.
- Preserved insight
- The person recognises the thoughts or urges as excessive or irrational, even though they cannot easily stop them — this is the key boundary against psychotic disorders.
- Compulsions reduce anxiety temporarily
- Rituals are performed specifically to reduce the anxiety an obsession causes, providing only short-lived relief.
- Resistance, at least initially
- The person generally attempts to resist or suppress the obsessions or compulsions, at least to some degree, even when unsuccessful.
How to weigh evidence
These learning labels are teaching aids, not official ICD-10 terminology (see Sources and Method).
- supports Recurrent, unwanted, intrusive thoughts recognised by the person as excessive, paired with repetitive rituals performed to reduce the resulting anxiety.
- consistent not decisive A person describes being 'a bit of a perfectionist' or 'liking things a certain way' without a clear obsession-compulsion anxiety-reduction cycle.
- raises alternative The person appears to genuinely believe the content of the thought is true and does not view it as excessive or irrational (lost insight).
- contradicts or weakens The thoughts are ego-syntonic (consistent with the person's own beliefs and values) rather than distressing and unwanted.
- missing and necessary to ask Whether the person recognises the thoughts as their own and as excessive or irrational (preserved insight), which is decisive against a psychotic disorder.
Diagnostic boundary
F42 requires an obsession-compulsion cycle (or obsessions or compulsions alone) with preserved insight — the person recognises the thoughts as unwanted, intrusive, and excessive, distinguishing this from a primary psychotic disorder where insight into the false or excessive nature of a belief is typically absent.
Full comparisons:
Duration and course
Minimum duration: Conventionally most days for at least two weeks, though many presentations are considerably more chronic.
Course: Often chronic and fluctuating in severity if untreated.
Included presentations
- Predominantly obsessional thoughts or ruminations without prominent compulsive rituals.
- Predominantly compulsive acts (obsessional rituals) without prominent obsessional thoughts.
- A genuine mixture of obsessional thoughts and compulsive acts, the most common clinical presentation.
Exclusions and distinguishing presentations
| If this is true instead… | Consider | Why |
|---|---|---|
| Loss of insight, with the person genuinely believing the thought's content is true | ocd intrusive thoughts equals psychosis | Preserved insight — recognising the thought as unwanted, intrusive, and excessive — is what distinguishes OCD from a primary psychotic symptom such as thought insertion or a delusion. |
| A fear reliably triggered by one specific external situation, without an internally-generated obsession-compulsion cycle | F40 | F40's phobias are triggered by an external situation; F42's obsessions and compulsions are internally generated and anxiety-reducing in function. |
Four-character subtypes (orientation)
A learner should not mistake an unusual or taboo intrusive thought for evidence of psychosis — the decisive check is always whether insight is preserved (the person recognises the thought as excessive, unwanted, and irrational), not the content of the thought itself.
- F42.0 — Predominantly obsessional thoughts or ruminations
- Obsessional thoughts without prominent compulsive rituals. (in_depth)
- F42.1 — Predominantly compulsive acts [obsessional rituals]
- Compulsive rituals without prominent obsessional thoughts. (in_depth)
- F42.2 — Mixed obsessional thoughts and acts
- A genuine mixture of obsessions and compulsions, the most common presentation. (in_depth)
- F42.8 — Other obsessive-compulsive disorders
- An obsessive-compulsive presentation not matching the named subtypes. (taught for orientation only, not separately assessed in this pilot)
- F42.9 — Obsessive-compulsive disorder, unspecified
- OCD identified but without enough information to specify 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.
External cause capsule — Organic, medical, or substance-related presentations resembling F40-F48
A reminder that a presentation resembling an anxiety, obsessive-compulsive, stress-related, dissociative, or somatoform disorder can instead be caused by a general medical condition (e.g. hyperthyroidism producing anxiety-like symptoms, a neurological condition producing genuine sensory or motor loss) or a substance/medication (e.g. caffeine or stimulant intoxication, sedative or alcohol withdrawal producing anxiety symptoms) rather than being a primary F40-F48 disorder, and that this should be actively considered before a primary F40-F48 classification is settled.
What to check:
- Timing: did the symptoms begin during or shortly after starting, stopping, intoxication with, or withdrawal from a substance or medication known to produce anxiety, dissociative, or somatic symptoms?
- Medical context: is there a known medical or neurological condition (e.g. thyroid disease, a cardiac arrhythmia, a neurological lesion) with a recognised association with the symptoms described?
- Adequacy of investigation: has an appropriate medical or neurological work-up actually been completed, rather than simply not yet undertaken?
- Course after the cause is addressed: symptoms that substantially improve once the medical condition is treated or the substance/medication is stopped weigh toward a secondary cause rather than a primary F40-F48 disorder.
A full teaching module on F00-F09 (organic mental disorders) or F10-F19 (substance-related disorders), or a general medical work-up, all of which are outside this capsule's scope.
Worked example — distinguishing OCD's intrusive thoughts from a psychotic symptom
A 24-year-old reports recurrent, distressing thoughts about accidentally harming a family member, which he finds horrifying and completely contrary to how he feels about them. He recognises the thoughts as irrational and not reflective of any actual wish to cause harm, but cannot stop them from intruding, and has started avoiding being alone with sharp objects near family members to reduce his anxiety, a ritual that brings brief relief before the thoughts return.
- Step 1 — Identify the obsession: recurrent, unwanted, distressing thoughts about harming a family member.
- Step 2 — Check insight: he recognises the thoughts as irrational and not reflective of an actual wish — insight is preserved.
- Step 3 — Identify the compulsion: avoidance of sharp objects near family, functioning as a ritual to reduce anxiety.
- Step 4 — Note the anxiety-reduction cycle: brief relief followed by the thoughts' return.
- Conclusion — F42.2, Mixed obsessional thoughts and acts — the preserved insight and anxiety-reduction cycle support OCD, not a psychotic disorder, despite the disturbing content of the thought.
Interactive practice
- Type A Checking the stove, again Recognition / application · Foundation
- Type E A thought he would never want Misconception correction · 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.
-
Define 'ego-dystonic' and explain why it matters for distinguishing OCD from a delusion.
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.
-
Explain the anxiety-reduction cycle between an obsession and a compulsion.
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.
-
Explain why disturbing thought content, on its own, does not indicate psychosis.
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.
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.
Rapid revision card
F42: F42 requires an obsession-compulsion cycle (or either alone) with preserved insight — the person recognises the thoughts as unwanted, excessive, and irrational.
Decisive boundary cues:
- Unwanted, ego-dystonic thoughts with preserved insight, paired with anxiety-reducing rituals -> weighs toward F42.
- Loss of insight, with the thought's content genuinely believed to be true -> weighs toward a psychotic disorder instead.
- A fear reliably triggered by one external situation, without an internal obsession-compulsion cycle -> weighs toward F40 instead.
Duration/course: Conventionally most days for at least two weeks; often considerably more chronic in practice.
Nearest alternatives: f40
Common mistake: Mistaking a disturbing or taboo intrusive thought for evidence of psychosis, without checking whether insight into its irrational, unwanted nature is preserved.
Retrieval prompt: Does the person recognise this thought as unwanted, excessive, and irrational (preserved insight), or do they believe its content is actually true?
(or use your browser's print function) Open full-page printable revision view →
Related glossary terms
-
Obsession (psychiatric)
An unwanted, intrusive thought, image, or urge that recurs persistently, is recognised by the person as their own but as excessive, senseless, or distressing (ego-dystonic), and that the person generally attempts to resist or suppress. Central to obsessive-compulsive disorder (F42).
-
Compulsion (psychiatric)
A repetitive behaviour or mental act that a person feels driven to perform, usually in response to an obsession, in order to reduce the anxiety or distress the obsession causes. The relief is typically brief, and the underlying obsession-compulsion cycle then repeats. Central to obsessive-compulsive disorder (F42).
Source basis
WHO ICD-10 international classification structure and code/title (ICD-10 Browser, 2019 English release).
Original educational synthesis, informed by general published clinical-education understanding of ICD-10 F40-F48; no verbatim CDDG or DCR text is reproduced. See Sources and Method for the full policy.
Last reviewed: 2026-09-16. Full policy: Sources and Method.
Educational resource only. This page and its cases teach ICD-10 classification reasoning using fictional scenarios. It does not diagnose real people and should not be used to assess yourself or anyone else. If you or someone you know is struggling with anxiety, intrusive thoughts, the effects of a stressful or traumatic event, or persistent physical symptoms, please seek prompt professional evaluation.