Substance Use Disorders (SUD) are often comorbid with psychological trauma, however, the complex interaction between the two is not yet fully understood. Most addiction-specialized professionals do not engage in exploring past traumatic experiences of the patients due to personal, professional, and educational barriers. Therefore, psychological trauma remains highly undetected and its contribution to the development and maintenance of SUD is neglected. This compromises the therapeutic results of most interventions, with relapse rates in SUD still remaining impressively high. EMDR is one of the most effective interventions for Post-Traumatic Stress Disorder (PTSD), and has been applied to other disorders that are often comorbid with trauma, such as psychosis and depression, with promising results. Nevertheless, its application in SUD is still limited. Taken altogether, there is a need to clarify the efficacy of Eye Movement Desensitization and Reprocessing (EMDR) therapy in SUD, as well as the mechanisms of action that mediate its potential therapeutic effects.

The aim of this study is to 1) determine the efficacy of EMDR therapy in patients with SUD comorbid with psychological trauma, as well as whether changes in these clinical variables correspond to changes in salivary cortisol levels- a robust marker of the Hypothalamic-Pituitary-Adrenal (HPA) axis; 2) investigate the mechanisms of action of EMDR therapy, paying special attention to the key role that the cerebellum might play in mediating its therapeutic effects.

Inclusion Criteria:

• A primary DSM-5 diagnosis of moderate to severe SUD, assessed by a clinical interview.
• No active drug consumption.
• Having experienced one or more traumatic events, assessed by the Life Event Checklist DSM-5 (LEC-5) and the Childhood Trauma Questionnaire (CTQ).
• Current posttraumatic symptoms, evaluated with the Impact Event Scale-Revised (IES-R).
• Aged from 18 to 65 years old.
• Sign an informed consent to participate in the study.
• Capable of speaking and comprehending Catalan or Spanish.

Exclusion Criteria:

• Having received a trauma-focused therapy within the last 5 years.
• Severe dissociative symptoms according to the Dissociative Experience Scale (DES).
• Presence of acute suicidal ideation.
• Acute episode of a comorbid psychiatric disorder.
• Severe cognitive impairments.
• Medical illness that compromises the HPA-axis.
• Long-term exposure to corticoids.
• Claustrophobia.
• Subjects with pacemakers.
• Presence of metallic objects within the body.
• Pregnancy