Key takeaways
- More than half of people in Canadian population studies report at least one adverse childhood experience (ACE); roughly 1 in 5 report three or more.
- Twin and family studies suggest substantial genetic liability to addiction, while environmental exposures—including childhood adversity, peer context, availability, stress, and social conditions—also contribute substantially to risk.
- For some people, childhood adversity may contribute to later substance use through difficulties with emotion regulation, avoidance, negative reinforcement, and using substances to manage distress.
- When trauma-related symptoms are actively maintaining substance use, treating the substance use without addressing those symptoms may leave an important relapse pathway untouched.
- Sequencing should be individualized. Acute intoxication, medically risky withdrawal, severe instability, or inability to participate safely may require substance-use stabilization first. But a person does not necessarily need prolonged abstinence before evidence-based PTSD treatment begins; when PTSD and SUD co-occur, concurrent trauma-focused and substance-use treatment can be appropriate.
For most of the last century, addiction was commonly framed as a failure of character or willpower. Research over the past two decades tells a more complicated story: for many people, compulsive substance use starts as a functional response to overwhelming internal states—and it works well enough, at least temporarily, which is part of why it becomes so difficult to stop.
How common is childhood adversity?
Adverse childhood experiences are more common than most people assume. A Public Health Ontario literature review notes that roughly half to two-thirds of participants in population-based studies report at least one ACE; in the Alberta ACE Study, 55.8% of participants reported one or more, and 20% reported three or more. A separate national analysis using Canadian Community Health Survey data found a 32% prevalence of childhood abuse among respondents, with a clear dose-response relationship between the number of adverse experiences and poorer mental health outcomes later in life.
The category covers what people typically expect—abuse, neglect, and exposure to violence—but also includes conditions that are harder to name: growing up in a household affected by a parent’s substance use or untreated mental illness, or a household coming apart through separation or incarceration.
ACEs are a population-level measure of adversity, not a diagnosis. Having ACEs does not necessarily mean someone was traumatized in a clinical sense or developed PTSD.
What counts as an adverse childhood experience?
Per the Public Health Ontario review, the commonly recognized list includes emotional, physical, and sexual abuse; emotional and physical neglect; growing up with a parent or caregiver who uses substances or has a mental health condition; exposure to intimate partner violence in the home; parental separation or divorce; and having a household member who has been incarcerated. Canadian researchers also increasingly recognize broader forms of adversity, including structural conditions like poverty and homelessness, and intergenerational and historical trauma affecting Indigenous communities.
The mechanism: toxic stress and a dysregulated nervous system
Exposure to ACEs during childhood can produce toxic stress—a prolonged stress response that affects developing brain architecture, compromises immune function, and increases vulnerability to poor health outcomes across the lifespan. Chronic adversity can shape developing stress-response and emotion-regulation systems, sometimes leaving a person more reactive to threat, slower to return to baseline, or more reliant on external strategies for regulating distress.
Research on trauma notes that a person who has experienced a traumatic event may develop either simple or complex post-traumatic stress and that complex PTSD tends to result from long-term, chronic trauma—often affecting a person’s ability to form healthy, trusting relationships going forward. In children, this pattern of chronic, complex trauma is sometimes referred to as developmental trauma.
That calibration doesn’t necessarily resolve when the circumstances that caused it end. The person may be safe now, but their stress-response system may still respond as though danger is close—or they may be left struggling with numbness, shame, disconnection, or difficulty regulating their internal state. If a substance reliably changes those states—by quieting distress, creating distance from painful emotions, or providing energy, relief, or connection—it can become a powerful form of regulation. Repeated relief reinforces the pattern, even as the longer-term consequences accumulate. Clinical research supports this observation: alcohol and other drugs are effective short-term anxiolytics and are often used to self-medicate symptoms of anxiety, which is part of why all patients presenting with a mood, anxiety, or psychotic disorder should be screened for substance use, and all patients with a substance use disorder should be screened for a history of trauma.
Viewed functionally, substance use may be doing something important for the person—reducing distress, dampening memories, creating distance from shame, facilitating connection, increasing energy, or producing temporary relief—even while causing serious harm.
Does trauma cause addiction?
Not on its own—addiction is multiply determined. Research describes addiction as a complex disorder involving the interaction of environmental factors with multiple genetic factors believed to increase a person’s susceptibility to addictive behaviour. Twin, adoptee, and family studies broadly support the view that a substantial share of addiction risk is genetic, with environment—including the childhood home environment—accounting for much of the remainder.
Childhood adversity is therefore best understood as an important risk factor rather than a singular cause. For some people, its ongoing effects—such as PTSD symptoms, shame, interpersonal threat sensitivity, avoidance, or emotion-regulation difficulties—remain clinically relevant to the maintenance of substance use and may need to be addressed in treatment.
Why treating substance use alone often falls short
CAMH uses the term “concurrent disorders” to describe co-occurring addiction and mental health problems, including trauma-related conditions. People often ask which came first—the mental health problem or the substance use—and CAMH notes this is often a hard question to answer, precisely because the two interact and reinforce each other over time.
This is part of the clinical rationale behind integrated treatment: making sure care for substance use and co-occurring mental health or trauma-related difficulties is coordinated rather than delivered in isolation. This understanding also informs the approach at Toronto Trauma & Addiction Counselling, where trauma and substance use can be addressed within the same treatment process. When substance use has become a way of managing an internal state that feels difficult to tolerate, simply removing the coping strategy may not be enough; treatment also needs to help the person develop other ways of doing the job the substance has been doing.
This framing also shifts the central clinical question. Alongside asking what keeps the substance use going, another clinically useful question is, what would stopping expose the person to?
What this means in practice
A few practical implications follow from this research:
- Assessment should include history, not just behaviour. Clinical best practices recommend screening every patient with a substance use disorder for a history of trauma, not just for co-occurring mood or anxiety symptoms.
- Sequencing matters. Stabilizing substance use first and addressing trauma “later” is sometimes clinically necessary, but as a default approach it can leave someone managing withdrawal with the coping mechanism removed and the underlying trauma untouched.
- Coordinated care matters. Integrated treatment models for concurrent disorders emphasize a stable, trusting relationship with one care coordinator, even when a team of professionals is involved—precisely because fragmented care tends to leave trauma unaddressed.
None of this makes recovery quick or simple. It does, however, make the process more legible, both to the person doing the work and to the people around them. For those navigating this path, understanding how to heal from past trauma can be a meaningful step forward.
Frequently asked questions
Does everyone with childhood trauma develop an addiction?
No—most people who experience childhood adversity do not develop an addiction. Adversity raises risk; it interacts with genetics, environment, and timing, and Canadian research shows a dose-response relationship where more adverse experiences correspond to greater risk, not a guaranteed outcome.
Is addiction a choice or a disease?
Neither framing fully captures it. Addiction is widely understood as a complex disorder shaped by both genetic susceptibility and environmental factors. Early use often involves choice; over time, the pattern can become one that no longer responds to willpower alone.
Can trauma therapy help someone who is still using?
Yes, in appropriate cases. A co-occurring substance-use disorder does not automatically require postponing evidence-based PTSD treatment until abstinence is achieved. Trauma-focused PTSD treatment can be delivered concurrently with substance-use treatment, although acute withdrawal, intoxication, severe instability, or other safety concerns may affect timing. The appropriate approach depends on the severity of the substance use, the person’s circumstances, and clinical judgment about safety.
What counts as an adverse childhood experience?
See “What counts as an adverse childhood experience?” above for the framework used in Canadian public health research, which covers both direct harm and household instability.
