Internet-based Treatment for Patients Suffering From Severe Functional Somatic Disorders

Inclusion Criteria: * Patients must fulfil the criteria for moderate-severe FSD operationalized as being diagnosed with either single/oligo-organ BDS, with 2 organ systems that are each affected by 3 or more symptoms, or multi-organ BDS, with 3 or 4 organ systems that are each affected by 3 or more symptoms. * Patients must have symptom duration \> 6 months. * If older than 25, patients must have been active in the labor market or educational system for at least 12 months during the last 2 years. * Patient must have access to a computer or tablet with internet connection. * Patient must be able to understand, read, and write Danish fluently. Exclusion Criteria: * Patients are excluded from participation if they present with severe comorbid somatic or psychiatric disease that is insufficiently treated at the time of assessment and is deemed to constitute a potential barrier for engaging in the treatment. * Patients are excluded if they lack motivation to engage in internet-delivered treatment. * Patients are excluded if they have poor self-reported IT skills. * Patients are excluded if they lack informed consent. * Patients are excluded if they have ongoing treatments with opioids and benzodiazepines. * Patient's use of psychoactive medications should be stable.