Defining Co-Occurring Disorders: An Overview of Complex Presentations
Co-occurring disorders (COD), also known as dual diagnosis, occurs when a person experiences a mental health disorder and a substance use disorder) simultaneously. These conditions interact in complex ways, often exacerbating each other. Mental health disorders include anxiety disorders, depressive disorders, bipolar disorder, PTSD and schizophrenia. Substance use disorders are considered an addiction to substances such as alcohol, opioids, and stimulants. The interaction between these disorders requires integrated treatment that addresses both the mental health and substance use at the same time. Accurate assessment and diagnosis is key to effective treatment, as misdiagnosis or not recognising one of the disorders can lead to ineffective or even harmful treatment.
Prevalence and Epidemiology of Co-Occurring Disorders

Co-occurring disorders are a public health issue. Epidemiology shows that people with a mental health disorder are more likely to get a substance use disorder and vice versa. According to the 2022 National Survey on Drug Use and Health (NSDUH), over 21.5 million adults and nearly one million adolescents (ages 12-17) in the USA were found to have a co-occuring Mental Health Issues and Substance Use Disorder, with fewer than half receiving some form of treatment. People who have experienced trauma, and those from lower socioeconomic backgrounds are at higher risk. There is a high societal impact of co-occurring disorders including healthcare costs, homelessness, and criminal behaviours. We need to understand co-occurring disorders prevalence and epidemiology to develop targeted prevention and intervention strategies.
Neurobiological Underpinnings of Co-Occurring Disorders
The co-occurrence of mental health and substance use disorders is often rooted in the same neurobiological mechanisms. Substance use can change brain structure and function, affect neurotransmitter systems and neural circuits involved in mood regulation, impulse control and reward processing. For example, chronic alcohol or drug use can disrupt the dopamine system and lead to anhedonia (inability to feel pleasure) and increases the risk of depression. Mental health disorders often affect the brain regions involved in substance use behaviors. People with anxiety disorders may use substances as a way to self-medicate their symptoms which can further alter brain chemistry and increase the risk of addiction. Both substance use disorders and mental health disorders often involve dysregulation in shared brain circuitry, including the prefrontal cortex (executive function), amygdala (emotional processing) and hippocampus (memory), which together contribute to impaired emotional regulation and altered reward processing
Trauma’s Role in the Development of Co-Occurring Disorders

Trauma plays a big role in the development of co-occurring disorders. People who have experienced trauma such as childhood abuse, neglect or exposure to violence are more likely to develop both mental health disorders and substance use disorders. Trauma can disrupt brain development, impair emotional regulation and increase vulnerability to stress. PTSD, a common mental health disorder after trauma, is often linked to substance use disorders as people may turn to substances to cope with intrusive memories, flashbacks and hyperarousal symptoms. The Adverse Childhood Experiences (ACEs) study has shown a strong correlation between childhood trauma and later life mental health and substance use problems. Trauma must be addressed in treatment for co-occurring disorders as unresolved trauma can undermine recovery and increase the risk of relapse.
Diagnostic Frameworks and Assessment Strategies for Co-Occurring Disorders

Co-occurring disorders diagnosis requires a full and thorough assessment that looks at both mental health and substance use symptoms to differentiate between mental health disorders, substance-induced symptoms or co-occurring disorders. The Diagnostic and Statistical Manual of Mental Disorders (DSM) has criteria for mental health disorders and substance use disorders that can be used to diagnose co-occurring disorders. Assessment tools include clinical interviews, psychological tests and substance use assessments, alongside a full historical assessment of mental health and substance use including onset, duration and severity of symptoms. Differential diagnosis is important to rule out other conditions that have similar symptoms such as substance induced mood disorders or primary mental health disorders with secondary substance use. Accurate diagnosis is the foundation for an effective and individualised treatment plan.
Integrated Treatment Models for Co-Occurring Disorders: A Holistic Approach
Integrative models are considered the gold standard for co-occurring disorders. These models acknowledge the co-occurrence of mental health and substance use and ensure that both conditions are treated at the same time through a coordinated integrative approach that combines therapies, interventions and support services. A multidisciplinary team will work together to design an effective treatment plan, including psychiatrists, psychologists, therapists and other health professionals.An integrative treatment strategy may include individual therapy, group therapy, medication management and psychoeducation.
Cognitive Behavioural Therapy (CBT) Adaptations for Co-Occurring Disorders
Cognitive Behavioural Therapy (CBT) is a widely used and effective therapy for mental health disorders and substance use disorders. When adapted for co-occurring disorders, CBT targets the specific cognitive and behavioural patterns that contribute to both conditions. CBT for co-occurring disorders may include cognitive restructuring to challenge negative thoughts and beliefs, relapse prevention strategies to manage cravings and triggers and coping skills training to improve emotional regulation and stress management. Clients learn to identify and modify maladaptive thoughts and behaviours that perpetuate both their mental health symptoms and substance use. CBT also helps individuals develop problem solving skills and build a support system for long term recovery.
Prolonged Exposure Therapy and EMDR in Trauma-Related Co-Occurring Disorders
For people with co-occurring disorders and a history of trauma, specialised therapies like Prolonged Exposure (PE) Therapy and Eye Movement Desensitisation and Reprocessing (EMDR) can be very effective. PE therapy involves gradually exposing clients to trauma related memories and situations in a safe and controlled environment to reduce anxiety and avoidance. EMDR helps clients process traumatic memories through bilateral stimulation, so they can reprocess and integrate those memories in a healthier way. Research suggests thatEMDR helps clients with adaptive information processing by reducing the emotional charge and promoting cognitive restructuring for emotional healing. Both PE and EMDR can help support trauma resolution, which can reduce reliance on substances as a coping mechanism, by addressing the traumatic triggers.
Relapse Prevention and Long-Term Management of Co-Occurring Disorders

Relapse prevention is a big part of long term management for people with co-occurring disorders. A relapse prevention plan involves identifying and managing triggers related to both mental health symptoms and substance cravings,with strategies required to address both areas. Ongoing therapy and support is crucial to maintain recovery and address any emerging issues. Lifestyle factors like stress management, sleep hygiene and social support also play a big role in preventing relapse. People with co-occurring disorders should be active in their recovery, set realistic goals and celebrate their progress. Long term management may also involve ongoing medication management and regular check ins with healthcare providers to monitor symptoms and adjust treatment as needed.
The combination of biomedical, psychological and environmental sciences is key to understanding and treating co-occurring disorders. By understanding how these factors work together clinicians can provide individualised care.
The Highlands Institute of Behavioural Medicine recognises the complexity of co-occurring disorders and the need for a whole of person approach to treatment. With a focus on behavioural medicine the centre aims to help clients achieve lasting recovery by addressing the underlying factors that contribute to both mental health and substance use issues. The four phase recovery program addresses the biological, psychological and social factors required for health and wellbeing.
Reviewed by: Dr. Emma Bardsley

Dr Emma Bardsley is a neuroscientist with a PhD from Oxford and a post doctorate from Auckland University, along with an undergraduate degree in Pharmacology from King’s College London. She has lectured extensively on neuroscience, physiology, and pharmacological interventions, bridging foundational research and its clinical applications. With a strong record of publications in high-impact journals and extensive experience in scientific writing, editing, and peer review, she excels at translating complex research into practical insights. Based in New Zealand and collaborating internationally, Emma is dedicated to advancing understanding and treatment in the fields of trauma, addiction, and recovery.
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