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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- Obsessions = recurrent, unwanted intrusive thoughts, images, or urges that cause marked anxiety
- Compulsions = repetitive behaviours or mental acts performed to reduce obsessional anxiety (e.g. handwashing, checking, counting)
- Y-BOCS (Yale-Brown Obsessive Compulsive Scale) for severity: 0–7 subclinical, 8–15 mild, 16–23 moderate, 24–31 severe, ≥32 extreme
- First-line: CBT with ERP (exposure and response prevention) — 10+ sessions with a specialist therapist
- If moderate-severe or CBT alone insufficient: SSRI (fluoxetine, fluvoxamine, sertraline) at HIGHER doses than depression
- Treatment-resistant: clomipramine (most effective but more side effects) or SSRI + low-dose antipsychotic augmentation
Overview
Obsessive-compulsive disorder (OCD) is characterised by obsessions (persistent, intrusive, unwanted thoughts, images, or urges that cause significant anxiety) and compulsions (repetitive behaviours or mental acts performed to reduce the anxiety caused by obsessions). Common themes include contamination (fear of germs → handwashing), harm (fear of causing harm → checking), symmetry (need for order → arranging), and taboo thoughts (intrusive sexual, violent, or blasphemous thoughts → mental rituals). Patients recognise their obsessions as irrational but cannot control them. OCD is classified separately from anxiety disorders in ICD-11 and DSM-5.
Epidemiology
OCD has a lifetime prevalence of approximately 1–2% in the UK. It affects men and women equally, although onset tends to be earlier in males (adolescence vs early adulthood). Mean age of onset is 20 years. There is a strong genetic component (concordance in monozygotic twins ~50%). Risk factors include family history, stressful life events, perinatal OCD (onset in pregnancy/postpartum), and comorbid anxiety or depression. OCD is frequently associated with depression (up to 70% comorbidity), body dysmorphic disorder, and tic disorders.
Clinical Features
Symptoms
Obsessional thoughts: contamination fears, harm/violence, sexual/blasphemous thoughts, symmetry/order, doubt
Compulsive behaviours: handwashing, checking (doors, appliances, gas), counting, ordering, hoarding
Mental compulsions: praying, counting, mentally reviewing, neutralising thoughts
Marked anxiety when unable to perform compulsions or when resisting obsessions
Time-consuming rituals (often >1 hour/day) causing significant distress and functional impairment
Avoidance of triggers (e.g. avoiding touching door handles, public toilets)
Insight: patient typically recognises thoughts are irrational but cannot control them
Poor insight or overvalued ideation (believing obsessions are true)
Signs
Dermatitis or skin damage from excessive handwashing (raw, cracked hands)
Visible anxiety, distress, or repetitive behaviour during consultation
Reassurance-seeking behaviour
Slow to complete tasks due to checking/ordering rituals
Investigations
First-line
Clinical diagnosisBased on history of obsessions and/or compulsions causing distress and functional impairment. Patient retains insight (distinguishes from psychosis)
Y-BOCSYale-Brown Obsessive Compulsive Scale: gold-standard severity measure. 10 items, each 0–4. Total 0–40. ≥16 = moderate, ≥24 = severe
Second-line
Screen for comorbidityDepression (PHQ-9) — present in up to 70%. Anxiety disorders, BDD, tics, eating disorders
Specialist
Neuropsychological assessmentIf diagnostic uncertainty — distinguish OCD from psychosis (insight preserved in OCD), OCPD, tic disorders
1
Mild functional impairment (Y-BOCS 8–15)
- Low-intensity CBT with ERP: guided self-help (workbooks/online) or brief individual CBT (up to 10 hours)
- Group CBT with ERP
2
Moderate functional impairment (Y-BOCS 16–23)
- Individual CBT with ERP: ≥10 therapist hours — must include ERP as the core component
- OR SSRI at HIGHER doses than used for depression (e.g. fluoxetine 40–60 mg, sertraline 150–200 mg, fluvoxamine 200–300 mg)
- Choose CBT with ERP over SSRI if available (more effective, no relapse on stopping)
3
Severe functional impairment (Y-BOCS ≥24)
- Combined CBT with ERP + SSRI (high-dose)
- If SSRI insufficient: switch to clomipramine (most effective anti-obsessional drug — max 250 mg/day, but more side effects and cardiotoxic in overdose)
- Augmentation: low-dose antipsychotic (risperidone or aripiprazole) added to SSRI — specialist decision
4
Treatment-refractory OCD
- Specialist OCD team referral
- Consider combined clomipramine + SSRI under specialist supervision (ECG monitoring required)
- Deep brain stimulation or neurosurgery (capsulotomy) in extreme, treatment-refractory cases — rare
Complications
- Depression: Up to 70% comorbidity — worsens OCD and vice versa
- Skin damage: Dermatitis, excoriation from compulsive handwashing or skin picking
- Social isolation: Rituals consume hours; shame about symptoms leads to concealment
- Relationship strain: Family accommodation of rituals perpetuates OCD
- Functional impairment: Unable to work, study, or maintain independence in severe cases
- Suicidal ideation: Elevated risk — always assess
UKMLA Exam Tips
- 1Obsessions = INTRUSIVE THOUGHTS the patient RESISTS. Compulsions = REPETITIVE ACTS to reduce anxiety from obsessions
- 2OCD patients retain INSIGHT — they know their thoughts are irrational. This distinguishes OCD from psychosis
- 3ERP (Exposure and Response Prevention) is the key component of CBT for OCD — expose to trigger, prevent compulsion
- 4SSRIs for OCD are used at HIGHER doses than for depression (e.g. fluoxetine 60 mg, sertraline 200 mg)
- 5Clomipramine is the most effective drug for OCD but is a TCA — higher side-effect burden and lethal in overdose
- 6OCD is ego-dystonic (distressing to the patient). OCPD is ego-syntonic (patient sees traits as desirable)
- 7Perinatal OCD: intrusive thoughts about harming the baby are COMMON and do NOT indicate actual risk of harm
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