Participants randomized to the intervention arm will attend VR sessions twice a week in 5 weeks, or 10 sessions in total. They will be placed into different virtual environments with increasing complexity, where they will be confronted with normal everyday situations in city environments.

In the first session, participants will receive instructions on using the head mounted display and navigating the scenarios. In three scenarios, participants are only observers, while in the remaining they can choose between several response options. These responses will be displayed on the screen, and participants can select their preferred response by pointing their finger to the chosen response alternative. The response alternatives are also provided verbally.

Treatment of substance use disorders (SUD) is delivered by personnel with medical, psychological, and social expertise, meeting specialist-level healthcare standards. SUD treatment clinics offer patient-centred, interdisciplinary, holistic treatment focusing on physical, mental and social problems. Treatment often consists of both individual and group therapies and include activities (e.g., physical exercise, social activities) in the clinic as well as in external settings. Since treatment approaches and facilities may vary across clinics, patients' preferences or choice of clinic are emphasized. According to national guidelines, persons with SUD and co-occurring severe mental illness (i.e., schizophrenia or severe bipolar disorder) can receive treatment in SUD treatment clinics as long as their mental condition is stable. Otherwise, persons with SUD and severe mental illness receive treatment for both conditions within mental health services.

The different VR scenarios comprise the following learning goals: 1) Social cognition. Interpreting social situations, understanding others; 2) Social communication. Express oneself, being understood by others; 3) Social competence. Interacting with others The investigators will use an MSI Raider GE78HX with a GeForce RTX4080 12GB GPU, running Windows 11, Unreal Engine 5.4, and the Meta Quest Link app, with a Meta Quest Pro as a PCVR headset connected via a Meta Quest Link cable. The setup works 100% locally, and no Wi-Fi or internet connection is used on the head-mounted-display or PC. The VR program itself does not store any data and will discard all that has happened inside the program, when restarted.

The primary outcome measure is disability measured with the 36-item WHO Disability Assessment Schedule (WHODAS 2.0) total score at 6 months post-intervention. WHODAS 2.0 measures health and disability in adults and captures the level of functioning in 6 domains: cognition, mobility, self-care, getting along, life activities and participation. The scores assigned to each of the items - "none" (1), "mild" (2) "moderate" (3), "severe" (4) and "extreme" (5) - are summed. Scores from each of the items are simply added up without recoding or collapsing of response categories; thus, there is no weighting of individual items. Higher sum score indicates more problems. WHODAS is based on the conceptual framework of the International Classification of Functioning. WHODAS is trans diagnostic, culturally sensitive and covers all diseases, including physical, mental and substance use disorders. It is is suitable for measuring the clinical effectiveness of interventions.

Measured by the Practical and Social Functioning Scale (PSF 3) consists of 32 items and 8 domains: personal hygiene, communication, managing economy, housekeeping, contact with social network, personal health care, transportation, and work and activities. Each item is rated on a five-point response scale from 1 (does not apply) to 5 (applies completely). Based on the completed form, scores for the different parts can be calculated as follows:

Total score for the entire scale with 28 items (sum 28 - 140, average score 1.0 - 5.0) Subscales (sum for each subscale 5 - 20, average score 1.0 - 5.0) Dimension activity (16 items, sum 16 - 80, average score 1.0 - 5.0) Dimension participation (12 items, sum 12 - 60, average score 1.0 - 5.0