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• Specific Disorders: Certain mental health disorders show particularly high rates of co-occurrence with SUDs. For instance, individuals with anxiety disorders are 2-3 times more likely to struggle with SUDs, while those with PTSD may be 2-4 times more likely. ADHD also significantly increases the risk of developing an SUD (Gateway Foundation, 2025). Among people entering alcohol and other drug treatment programs in Australia, between 50–76% meet the diagnostic criteria for at least one comorbid mental illness (AIHW, 2025). Half of people living with psychotic illness (51%) had a lifetime history of harmful alcohol use or dependence, double the rate in the general population (25%), and over half (55%) had a lifetime history of harmful illicit drug use or dependence – 6 times the rate of the general population (9%) (AIHW, 2025).

How Co-Occurring Disorders Complicate Assessment and Treatment

The presence of co-occurring disorders significantly complicates the assessment and treatment process due to several interacting factors:

1. Symptom Overlap and Masking: Many symptoms of substance intoxication or withdrawal can mimic mental health disorders, and vice versa. For example, anxiety and paranoia can be symptoms of both stimulant withdrawal and an anxiety disorder. Depression can be a symptom of chronic substance use or a primary mood disorder. This overlap makes accurate diagnosis challenging, as it’s difficult to discern which symptoms are primary and which are substance-induced or exacerbated. Misdiagnosis can lead to ineffective treatment strategies.
2. Mutual Exacerbation: The two conditions often feed into each other in a vicious cycle. Mental health symptoms (e.g., anxiety, depression, trauma) can drive individuals to self-medicate with substances, providing temporary relief but ultimately worsening both conditions. Conversely, chronic substance use can alter brain chemistry, trigger or worsen mental health symptoms, and interfere with the effectiveness of mental health medications. This mutual reinforcement makes recovery more difficult if only one disorder is addressed.
3. Treatment Engagement and Compliance: Individuals with co-occurring disorders often experience more severe symptoms and greater functional impairment compared to those with a single diagnosis. This can lead to lower motivation for treatment, higher rates of treatment dropout, and poorer compliance with medication or therapy regimens. The fluctuating nature of mental health symptoms can also make consistent engagement in SUD treatment challenging.
4. Increased Complexity and Relapse Risk: The presence of two complex conditions means treatment plans need to be highly individualized and comprehensive. Triggers for substance use might be intertwined with mental health symptoms, making relapse prevention more intricate. For example, a panic attack might trigger a craving for alcohol, or a depressive episode might lead to increased drug use.
5. Stigma and System Silos: Historically, mental health and substance use treatment systems have operated independently, creating barriers to integrated care. Patients might be denied treatment in one system if they also present with the other condition, leading to fragmented or no care. Stigma associated with both conditions can also deter individuals from seeking help or openly discussing all their symptoms.