Comorbidity in ADHD and autism

By Markus Lien, clinical psychologist · February 27, 2026 · Last updated: March 2026

Most people diagnosed with ADHD or autism do not have only that one condition. In a large American study, Kessler et al. (2006, Archives of General Psychiatry) found that 70–80% of adults with ADHD met criteria for at least one additional psychiatric diagnosis. For autism, the picture is equally complex. Yet many are still assessed for a single diagnosis at a time, as if the mind were divided into separate drawers.

This article is about comorbidity: the conditions that live alongside ADHD and autism, how they mask each other, and why an assessment that only looks for one thing risks finding the wrong one.

We walk through the most common comorbid conditions, explain how Alethia screens for all of them, and describe the clinical decision process that determines whether a condition is an alternative to ADHD, an independent co-occurring diagnosis, or a secondary consequence of untreated ADHD.

Comorbidity is the rule, not the exception

The word comorbidity simply means that two or more conditions occur in the same person. It sounds technical, but the implications are enormous. If you have ADHD, the probability that you also have another psychiatric condition is far greater than the probability that you only have ADHD.

This is not a rare exception. Having another diagnosis alongside ADHD is the norm, not the outlier. Many have two or three. The numbers for autism are comparable: around 70% have at least one comorbid condition, and over 40% have two or more.

Children with ADHD are 10 times more likely to have ODD (oppositional defiant disorder) or conduct disorder, 5 times more likely to have depression, and 3 times more likely to have anxiety compared to children without ADHD (Larson et al., 2011, Pediatrics). These numbers are not coincidences. They reflect deep neurobiological overlaps.

Looking specifically at ADHD and autism, co-occurrence rates range from 30 to 80% in clinical samples, depending on the population (Rommelse et al., 2010, European Child & Adolescent Psychiatry). The range is wide, but the conclusion is the same: the two conditions co-occur far more frequently than previously assumed.

Under ICD-10, it was actually not permitted to assign both diagnoses simultaneously. Autism was considered hierarchically superior, “excluding” ADHD. This meant that thousands of people with both conditions only had one recognized. ICD-11, which Denmark transitions to in 2027, now permits the dual diagnosis. It is one of the most important changes in the new classification.

The most common comorbid conditions

The comorbidity profile changes with age. In children, behavioral disorders and learning difficulties dominate. In adults, it is mood disorders, anxiety, substance use, and personality factors.

Childhood and adolescence

Adulthood

When children with ADHD become adults, the comorbidity picture shifts. The conduct disorder from childhood can develop into substance use and personality difficulties. The depression that started as a reaction to school difficulties can become chronic. New comorbid conditions emerge.

Medical comorbidities

Comorbidity is not limited to psychiatry. ADHD is associated with a range of medical conditions: obesity, asthma and atopic conditions, epilepsy, and diabetes. The mechanisms are not fully understood, but research points to chronic immune dysregulation and inflammatory processes as possible connecting factors. This is not something we assess in a psychological evaluation, but it underscores that ADHD is not an isolated brain thing. It is a condition with consequences throughout the body.

How comorbid conditions mask each other

One of the most serious problems with comorbidity is diagnostic shadow. One condition hides behind the other. A person with ADHD and anxiety may receive the anxiety diagnosis because anxiety is the most visible thing in the consultation room. But the anxiety may well be secondary: triggered by 30 years of struggling with attention problems, missed deadlines, and a constant feeling of falling short.

Depression is another common mask. Many adults with undiagnosed ADHD have lived with a depression diagnosis for years. They received SSRIs that did not really help, because the underlying cause wasn't identified. When ADHD is treated, the depression often lifts because it was reactive.

Autism can mask ADHD, and ADHD can mask autism. A person with autism may appear attentive because their intense special interests resemble concentration. A person with ADHD may appear socially flexible because their impulsivity is mistaken for spontaneity. When both conditions are present, they can partially cancel out each other's most visible features.

Personality factors complicate the picture further. Under ICD-11, we use a dimensional model for personality, and the Disinhibition domain directly overlaps with ADHD impulsivity. Without thorough assessment, one can confuse ADHD-driven impulsivity with personality traits, or vice versa.

The assessment paradox

There is a deep paradox in psychiatric assessment that is rarely discussed: The consultation room itself creates the precise conditions that suppress what we are trying to observe.

Novelty stimulates dopamine. When a person with ADHD sits in a new situation, with a new person, in a new room, dopamine levels temporarily rise. This means the patient may appear more focused in the consultation than in everyday life. One-on-one attention from the clinician provides external structure, the precise thing the patient lacks in daily life. The compensatory strategies the patient has built over decades can be read as competence rather than as signs of how hard they work to maintain a facade.

Self-report instruments capture the mask, not the face behind it. A person who has learned to compensate effectively may score low on ADHD screenings, not because they do not have ADHD, but because they have learned to navigate the world despite it.

The solution is not to abandon screening instruments. The solution is to supplement them with broad screening, targeted probing, longitudinal history (the life story, not just the current presentation), and collateral information from informants who knew the patient as a child. You cannot trust the cross-sectional picture alone.

How Alethia screens: The two-stage model

At Alethia, we assess six areas: ADHD/ADD, autism, personality structure, trauma, anxiety, and depression. Our assessment process uses a systematic screening across all psychiatric categories (ICD-10 F0 through F9) that casts a wide net across the entire psychiatric spectrum. No condition is missed because we were not looking for it.

The process follows two stages.

Stage 1: Broad screening (part of the first session, approx. 2 hours)

In the first session, we conduct a broad screening that covers all relevant psychiatric categories. Here are the key ones:

Stage 2: Targeted probing

When screening flags something, we go deeper. Stage 2 is not automatic. It is activated only when stage 1 shows signs that require further clarification.

Differential diagnosis, comorbidity, or secondary condition?

For each condition flagged in screening, we must answer a crucial question: What is the relationship between this condition and ADHD? There are three possibilities, and they lead to completely different treatment plans.

1. Alternative explanation (differential diagnosis)

The condition explains the symptoms BETTER than ADHD. The patient does not have ADHD, but something that resembles it.

Example: A person with bipolar type 2 can appear restless, distracted, and impulsive during hypomanic phases. It resembles ADHD. But the difference is that it is episodic: it comes and goes. ADHD is chronic and present from childhood. If symptoms are only present in distinct periods, bipolar is the more likely explanation.

2. Independent co-occurring condition (comorbidity)

The condition exists independently of ADHD. Both are present, both are real, and both require independent treatment.

Example: A person with ADHD and autism. The autism is not caused by ADHD. ADHD is not caused by autism. They are two independent neurodevelopmental conditions that happen to occur in the same person. Both require recognition and adapted approaches.

3. Secondary consequence of ADHD

The condition is TRIGGERED by untreated ADHD. It is real, but it is a consequence, not an independent cause.

Example: A person with ADHD and generalized anxiety. The anxiety began in their 20s, after a decade of missed deadlines, failed studies, and social isolation. The anxiety is real and distressing. But it is secondary: triggered by the chronic stress that untreated ADHD has caused. When ADHD is treated, the anxiety often eases significantly because its source disappears.

This three-way distinction is crucial because the treatment plan is fundamentally different. If the anxiety is a differential diagnosis, we treat the anxiety. If it is comorbid, we treat both in parallel. If it is secondary, we start with ADHD and see if the anxiety lifts.

Treatment sequencing: What comes first?

When an assessment reveals multiple concurrent conditions, the next question is: In what order should they be treated? The answer depends on the severity of the conditions and how they interact.

Acute conditions first

Psychosis, current mania, suicidal crisis. These conditions always take priority, regardless of what else is in play. Stimulant medication is not initiated until the patient is stabilized.

Mood stabilization before stimulants

If there are signs of bipolar disorder (approximately 20% comorbidity with ADHD), mood is stabilized first. Stimulants can potentially trigger a manic episode in a person with untreated bipolar. Only when mood is stable can ADHD medication be considered, often in combination with mood stabilizing medication.

Trauma may take priority

Complex PTSD can undermine the validity of the assessment itself. If a person is in a state of chronic hyperarousal and emotional dysregulation due to untreated trauma, it can be difficult to determine whether the underlying attention problems are due to ADHD or are trauma-related. In such cases, trauma treatment (e.g., EMDR or trauma-focused therapy) may be necessary before the ADHD assessment can be meaningfully completed.

ADHD and anxiety can be treated in parallel

A common misconception is that anxiety is a contraindication for stimulant medication. It is not. Many people with ADHD and anxiety actually find that their anxiety decreases when ADHD is treated, because the anxiety was secondary. Stimulants do not necessarily increase anxiety. For some, it decreases because the cognitive calm that medication provides reduces chronic stress.

ADHD and autism: When two developmental conditions meet

The co-occurrence of ADHD and autism deserves its own section because it is so frequent and because it has historically been underdiagnosed.

Under ICD-10, it was not permitted to assign both diagnoses simultaneously. Autism was hierarchically superior: If a child met autism criteria, ADHD could not be added. This meant that many people with both conditions only had one recognized. ICD-11, which Denmark transitions to in 2027, removes this restriction. Internationally, clinical practice has long diagnosed both, and the research literature is clear: 30 to 80% of people with one condition also meet criteria for the other.

ADHD and autism can resemble each other on the surface. Both can involve difficulties with attention, executive function, and social interaction. But the mechanisms are different. A person with ADHD misses social cues because they are distracted. A person with autism misses them because they do not intuitively read them. A person with ADHD shifts focus constantly. A person with autism may have difficulty shifting focus at all.

When both conditions are present, it is crucial to identify both because the treatment plan differs. Stimulant medication can help with ADHD symptoms (see our ADHD assessment), but it does not help with autism's core features. Conversely, the accommodations that help with autism (predictability, structured environments) can partially compensate for ADHD symptoms. Our autism assessment ensures both are identified.

What this means for you

If you are considering an assessment for ADHD or autism, a few things are worth knowing.

A thorough assessment does not only look for what you expect. Many come to us suspecting ADHD, and it turns out there is also anxiety, autism traits, or personality factors that change the picture. Others come with a depression diagnosis they have had for years and discover that ADHD was the underlying cause all along. The assessment takes as long as it takes. Some have an uncomplicated ADHD profile that can be clarified in 3 sessions. Others have a complex picture requiring additional investigation. That is why we screen broadly in the first session.

The cost depends on what is needed. A full ADHD assessment with medication initiation costs 14,300 DKK (6 sessions). An autism assessment costs 9,400 DKK without ADOS-2 or 12,900 DKK with ADOS-2. A combined ADHD and autism assessment with medication initiation costs approximately 17,800 DKK. You pay per session over several weeks, not all at once. The first session (clinical interview and broad screening) costs 2,900 DKK.

What it does to self-understanding

The clinical side of comorbidity is about finding the right diagnoses and the right treatment sequence. But there is a psychological dimension that is rarely discussed: What does it do to a person to discover that it is not one, but two or three conditions that have shaped their entire life?

For some, it is a relief. Multiple diagnoses provide more answers. Anxiety that felt like a personality trait may turn out to be secondary to ADHD. Social exhaustion that looked like introversion may turn out to be autism. Suddenly the puzzle comes together in a way it didn't before.

For others, it is overwhelming. Three diagnoses can feel like three different things wrong with you, especially if the internal narrative has been about 'managing' and 'holding it together'. It is in that moment that psychoeducation becomes crucial: understanding that the diagnoses are not three separate faults, but three parts of one picture. Comorbidity is not three diseases. It is one person whose brain works in a particular way, with consequences that extend in several directions.

That process takes time. Many need to sit with the new picture for weeks or months before it starts to make sense. That is one of the reasons our feedback session is not just a report being delivered. It is a conversation about what the results mean for you specifically, your life, and your self-understanding.

Why broad screening matters more than a single diagnosis

Many clinics assess one condition at a time. You are referred for ADHD, they assess for ADHD, and you either get the diagnosis or not. It sounds logical, but it ignores the fact that few people have ADHD alone. Most carry other conditions alongside it, conditions that affect the symptom picture, treatment choice, and prognosis.

When we screen broadly, we gain three advantages. First, we catch differential diagnoses, conditions that resemble ADHD but are not. Second, we identify comorbid conditions that require independent treatment. Third, we clarify which symptoms are secondary to ADHD and can be expected to improve when ADHD is treated.

That is the difference between giving a person a diagnosis and giving a person a picture of what is going on. The first can be wrong. The second provides the foundation for a treatment plan that actually works.

References and further reading

• WHO ICD-11: ADHD (6A05). The International Classification of Diseases, 11th revision. Now permits dual diagnosis of ADHD and autism.
• Danish Health Authority: National clinical guideline for ADHD in adults. Danish recommendations for ADHD assessment and treatment.
• World Federation of ADHD. International consensus and research on ADHD, including comorbidity.