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David Baxter PhD

Late Founder

What Is Complex ADHD? Definition, Diagnosis & Treatment

by Theresa Cerulli, M.D., ADDitude
July 16, 2021

The term “complex ADHD” reflects an evolution in our understanding of the condition, its scope, and its common co-occurrence with one or more psychiatric, learning, or other neurodevelopmental disorders.

Research confirms that attention deficit hyperactivity disorder (ADHD or ADD) commonly co-occurs with other conditions. In fact, we might say this is the rule rather than the exception. As many as 80% of adults with ADHD have at least one co-existing psychiatric disorder1, while approximately 60% of children with ADHD have at least one co-occurring condition2.

Common conditions co-occurring with ADHD include:
Apart from conveying comorbidities, the term complex ADHD also describes the condition’s heterogeneity and the variety of factors that can influence its presentation. It also reflects ADHD’s known impact on functioning across many domains of life, especially when symptoms are not adequately treated.

The presence of co-occurring conditions almost always muddles the diagnosis, treatment, and prognosis of ADHD. ADHD and comorbidities may also influence the presentation and severity of one another, which can complicate the detection and treatment of symptoms, and impair overall quality of life. (Serious outcomes are also associated with comorbid conditions. According to one study, mortality risk – already elevated for individuals with ADHD – increases substantially with the number of psychiatric comorbidities3. For all these reasons, recognizing ADHD’s “complexity” is of high clinical importance.

Complex ADHD: Background on Terminology​

The medical community has long recognized the high rates of comorbid conditions among patients with ADHD. Recent updates to ADHD diagnosis and treatment guidelines from professional associations, however, further underscore the importance of considering co-existing conditions with ADHD:
  • 2019: The American Academy of Pediatrics (AAP), in its updated guidelines for the diagnosis, evaluation, and treatment of ADHD in children and adolescents, recommends that clinicians trained in diagnosing comorbid conditions initiate treatment for such conditions, or, if inexperienced, refer patients to specialists.
  • 2020: The Society for Developmental and Behavioral Pediatrics (SDBP), to complement the AAP’s updated guidelines, publishes its own guidelines for assessing and treating children and adolescents with “complex ADHD.” These guidelines recommend that trained clinicians assess for and develop multimodal treatment plans for complex ADHD.

Understanding Complex ADHD​

ADHD Comorbidities Change with Age​

Although ADHD is associated with various co-occurring conditions, prevalence rates for comorbidities tend to change as an individual ages. For example:

In Children​

  • Behavior and conduct problems, like ODD and conduct disorder, occur in about half of children with ADHD2, and are not as common in adults with ADHD

In Adults​

What Explains ADHD Comorbidity Rates?​

It is believed that the co-occurrence of ADHD and comorbid conditions arises partly from shared underlying neuropsychological dysfunctions6.

ADHD’s heterogeneous presentation, as well as the parts of the brain implicated in ADHD, may explain why comorbidities are not only frequent, but also wide ranging.

The prefrontal cortex (PFC), which regulates attention, behavior, and impulsivity, features prominently in the neurological underpinnings for ADHD. But not all parts of the PFC – or the neural networks of the brain, for that matter – are impacted in the same way, which explains why ADHD can be so different from person to person.

The variety in ADHD symptom presentation and an individual’s associated features, combined with considerable duplication with symptoms of co-occurring conditions, has led experts to conceptualize ADHD as a spectrum disorder1.

Complex ADHD: Diagnosis​

Diagnosing complex ADHD starts with confirming that a patient meets DSM-5 criteria for ADHD alone. Children with ADHD need to exhibit six or more symptoms of inattention and/or hyperactivity and impulsivity to merit a diagnosis. Adults need only exhibit five symptoms. Learn more about DSM-5 symptoms and full diagnostic criteria here: What is ADHD, and how is it diagnosed?

Conducting a thorough ADHD evaluation is a multi-step process that may involve the use of diagnostic rating scales like the ADHD Rating Scale-5, the Vanderbilt Parents and Teacher, and the Conners Parent Rating Scale.

Even if a patient does exhibit symptoms of ADHD, clinicians should still rule out alternative explanations as part of the evaluation. Changes to the patient’s home environment and circumstances, for example, could influence symptom severity and presentation.

Regardless of whether an ADHD diagnosis is established, clinicians should assess for comorbid conditions. The diagnosing clinician, if experienced in doing so, can carry out assessment for other conditions. Otherwise, they should refer the patient to an appropriate subspecialist.

As with ADHD, clinicians may assess for comorbidities by using diagnostic rating scales like the Patient Health Questionnaire, Mood Disorder Questionnaire, and Social Responsiveness Scale-2.

While complex ADHD is generally defined as ADHD with a co-occurring condition, the SDBP notes that complex ADHD is defined by any of the following:
  • The presence of suspicion of: co-existing disorders and complicating factors; neurodevelopmental disorders; specific learning disorders; mental health disorders; medical conditions; genetic disorders; complicated psychosocial factors; and/or functional impairments
  • Diagnostic uncertainty on the part of the primary care clinician
  • Inadequate response to treatment
  • The patient is younger than 4 or older than 12 years of age at the time of initial presentation of symptoms

Complex ADHD: Treatment​

The accepted approach to addressing complex ADHD is to treat the comorbidities first only if they are severe, and, in all other cases, to treat ADHD and the comorbidities simultaneously. This is what makes treating complex ADHD a delicate balancing act – one condition can’t be ignored for the other(s). Treating ADHD may resolve and improve co-existing conditions. However, comorbidities may also require separate treatment. It is also true that treating just one condition can worsen others.

This updated paradigm differs from the previously accepted approach of treating comorbidities first, and then treating ADHD.

Treating ADHD symptoms in an individual with complex ADHD should follow a multimodal approach that may include:
  • Pharmacotherapy: Stimulants are first-line medications for the treatment of ADHD, followed by non-stimulants, or sometimes a combination of both.
  • Psychotherapy (individual, couples, and/or family) may also help manage co-existing conditions
  • ADHD and executive function coaching
  • Behavioral parent training (for children with ADHD)
  • Academic and/or workplace accommodations
  • Individual and family supports
  • Healthy habits – nutrition, sleep, and exercise
Clinicians should work with patients to determine unique areas of impairment and difficulty, and tailor treatments accordingly. At the start of treatment, patients should list goals and symptoms to target for improvement. Over time, the patient should note (perhaps using a scale model) how close (or far) they have moved with each symptom.

Complex ADHD: Next Steps​


1 Katzman, M. A., Bilkey, T. S., Chokka, P. R., Fallu, A., & Klassen, L. J. (2017). Adult ADHD and comorbid disorders: clinical implications of a dimensional approach. BMC psychiatry, 17(1), 302. Adult ADHD and comorbid disorders: clinical implications of a dimensional approach - BMC Psychiatry

2 Melissa L. Danielson, Rebecca H. Bitsko, Reem M. Ghandour, Joseph R. Holbrook, Michael D. Kogan & Stephen J. Blumberg. (Jan. 24, 2018). Prevalence of Parent-Reported ADHD Diagnosis and Associated Treatment Among U.S. Children and Adolescents, 2016. Journal of Clinical Child & Adolescent Psychology, 47:2, 199-212, DOI: 10.1080/15374416.2017.1417860. Retrieved from:

3 Sun S, Kuja-Halkola R, Faraone SV, et al. (2019). Association of psychiatric comorbidity with the risk of premature death among children and adults with attention-deficit/hyperactivity disorder. JAMA Psychiatry, 76(11):1141–1149. doi:10.1001/jamapsychiatry.2019.1944

4 Managing ADHD in children, adolescents, and adults with comorbid anxiety in primary care. (2007). Primary care companion to the Journal of clinical psychiatry, 9(2), 129–138.

5 Wilens, T. E., & Morrison, N. R. (2012). Substance-use disorders in adolescents and adults with ADHD: focus on treatment. Neuropsychiatry, 2(4), 301–312.

6 Rommelse, N.N.J., Altink, M.E., Fliers, E.A. et al. (2009). Comorbid problems in ADHD: Degree of association, shared endophenotypes, and formation of distinct subtypes. Implications for a future DSM . J Abnorm Child Psychol, 37, 793–804 .
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