ADHD and obsessive–compulsive disorder (OCD) are sometimes discussed as closely related conditions. While they can co-occur, research shows that their relationship is complex, uneven, and often misunderstood.

This article explores how ADHD and OCD overlap, how they differ, and why simplistic claims about their connection are misleading.

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What Is OCD?

Obsessive–compulsive disorder involves two core symptom types:

Obsessions

Obsessions are unwanted, intrusive, and repetitive thoughts, images, or urges that cause significant anxiety or distress. Common examples include:

• Fear of contamination or germs
• Fear of losing important items
• Aggressive or violent thoughts
• Unwanted sexual or religious thoughts
• A strong need for symmetry or exactness

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Compulsions

Compulsions are repetitive behaviours or mental acts performed to reduce anxiety caused by obsessions. These can include:

• Excessive cleaning or handwashing
• Repeated checking
• Counting rituals
• Ordering or arranging objects
• Hoarding behaviours

Obsessions are thoughts; compulsions are actions — a distinction that matters when comparing OCD to ADHD.

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How Often Do ADHD and OCD Co-Occur?

OCD affects around 2–3% of the population, while ADHD is more common, affecting roughly 5–8% overall (with higher rates in childhood and lower rates in adulthood).

Looking at overlap:

• Among people with OCD, around 12–25% also meet criteria for ADHD
• This means individuals with OCD are 2–5 times more likely to have ADHD than the general population
• However, most people with OCD do not have ADHD

When the direction is reversed:

• Only about 4–8% of people with ADHD have OCD
• In some long-term studies, OCD rates in ADHD populations are no higher than average

The key point: most people with either condition do not have the other.

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A “Lopsided” Comorbidity

The relationship between ADHD and OCD is not symmetrical.

• Having OCD increases the likelihood of also having ADHD
• Having ADHD only slightly increases the likelihood of having OCD

This pattern is sometimes described as a lopsided comorbidity. It suggests that shared mechanisms are limited and that different factors may be influencing each disorder.

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The Role of Tic Disorders

OCD is strongly associated with tic disorders and Tourette syndrome, which are also commonly linked with ADHD. This raises an important unanswered question:

• How much of the ADHD–OCD overlap is actually explained by tic-related conditions?

Few studies have fully controlled for this factor, but it is likely that doing so would reduce the apparent overlap.

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Different Comorbidity Patterns

The broader comorbidity profiles of ADHD and OCD are quite different:

• ADHD commonly co-occurs with oppositional disorders and learning disabilities in childhood, and with anxiety, depression, substance use, and antisocial traits in adulthood
• OCD shows a different pattern, often involving anxiety-related and tic-related conditions

This again suggests that ADHD and OCD are not variations of the same underlying disorder.

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Brain Differences: Same Regions, Opposite Patterns

Both ADHD and OCD involve similar brain regions, including:

• The prefrontal cortex
• The basal ganglia
• The thalamus
• The cerebellum

However, the direction of change is often opposite:

• In OCD, certain frontal and basal ganglia regions tend to be larger and overactive
• In ADHD, these same regions are often smaller and underactive

The anterior cingulate cortex, in particular, shows increased activity in OCD and reduced or normal activity in ADHD.

These opposing patterns support the idea that the two disorders sit at opposite ends of a control spectrum.

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Impulsivity vs Compulsivity

Conceptually:

• ADHD aligns more closely with impulsivity
• OCD aligns more closely with compulsivity

This distinction is reflected in both behaviour and brain function, even though some overlapping regions are involved.

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Neurochemistry and Medication Differences

The two disorders also differ neurochemically:

• OCD is associated with increased dopamine activity and reduced serotonin regulation
• ADHD involves reduced dopamine and norepinephrine activity

These differences explain why treatments differ:

• SSRIs are effective for OCD but not ADHD
• Stimulants and norepinephrine reuptake inhibitors help ADHD but not OCD
• In some cases, stimulants may worsen OCD symptoms

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Genetics: Both Heritable, But Not Identical

Both disorders are highly heritable:

• ADHD: 60–88% of variation explained by genetics
• OCD: 45–65% genetic contribution

ADHD shows stronger genetic influence overall, while OCD appears to involve a modest contribution from shared family factors as well.

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A Possible Source of Misdiagnosis

One important complication in this research is the lack of attention to cognitive disengagement syndrome (CDS).

CDS involves:

• Internal preoccupation
• Mental fogginess
• Daydreaming and slowed thinking

This profile may overlap more naturally with OCD than with classic ADHD. However, CDS is rarely diagnosed and is often misclassified as inattentive ADHD.

This raises the possibility that:

• Some reported ADHD–OCD overlap reflects misdiagnosed CDS, not ADHD

At present, this has not been adequately studied.

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Key Takeaways

• ADHD and OCD can co-occur, but it is uncommon
• The overlap is uneven, not reciprocal
• Both disorders involve similar brain regions but show opposite patterns of activity
• They differ in neurochemistry, genetics, and treatment response
• ADHD aligns more with impulsivity, OCD with compulsivity
• Some overlap may reflect diagnostic limitations rather than true comorbidity

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Final Thought

ADHD and OCD are distinct conditions with different mechanisms, even when they appear together. Treating them as closely related or interchangeable oversimplifies the science and risks misunderstanding both.

Nuance matters — especially when it comes to neurodevelopmental and psychiatric conditions.