The primary aim of this study is to investigate the effect of Internet-delivered Acceptance and commitment therapy for PTSD and comorbid chronic pain using a randomized controlled trial with waitlist control.

Objective: The primary aim of this study is to investigate the effect of Internet-delivered Acceptance and commitment therapy for PTSD and comorbid chronic pain.

Sample size : 40 (20+20) participants in total.

Trial design: A randomised trial where participants are allocated to either IACT for PTSD/Pain or control group (wait-list 12 weeks). All participants will eventually receive treatment, since participants randomized to the control group are subsequently offered identical treatment.

Participants are recruited from the Pain Rehabilitation Unit at Skåne University Hospital. The unit is a government supported, regional specialist center focused on assessment and treatment of chronic pain and related disability.

Assessments: Baseline and post treatment (2 weeks after treatment) assessments will be conducted by an assessor who is trained to administer the study measures. Self-report measures will also be collected at this time, post treatment (2 weeks after treatment) as well as during a 3-month and a 12-month follow up.

Assessment includes:

Pre-and post assessment: Assessors will collect demographic information, self-report measures, and trauma history.

During the pre-assessment the Clinician-Administered PTSD Scale for DSM-5 will be administered to establish that the participants meet the DSM-5 criteria for PTSD. The Mini International Neuropsychiatric Interview 7.0 (MINI) will also be administered to detect the presence of other comorbid disorders and assess inclusion criteria and rule out exclusion criteria.

Post-treatment exit interview: At the post-assessment, the assigned assessor will ask participants about their satisfaction with and experience of the program, what they found helpful or unhelpful and suggestions for future improvements.

During treatment: During treatment the treatment credibility scale will be administered to assess the patients' perceptions of how credible the treatment is following the introduction of the treatment rationale and the main treatment components (included in the internet program).

Safety parameters: As a mean to monitor safety and progress participants complete two self-report measures within the program (PTSD Checklist for DSM-5 [PCL-5] and Hospital anxiety and depression scale [HADS] twice during the program and the therapist can follow these scores. In addition, participants can report any adverse events during treatment, at post treatment and follow-up assessment.

Data collection: Self-report measures will be mailed to participants

Main statistical analysis: Between-group estimates on outcome will be conducted using repeated measurements. The analyses will be conducted using intention to treat principles and post hoc comparisons.

Inclusion Criteria:

• a CAPS of ≥25
• subjected to single traumatic events
• were able to understand Swedish
• had symptoms of chronic pain that interfered significantly with everyday life
• were fully examined medically and had received medical treatment if indicated
• were able to be an active part of the rehabilitation process, regain functioning in different life areas and participate in treatment interventions for approximately 5 hours every week
• stable dose of medication
• able to read and write in Swedish
• had access to a smart phone or computer with internet access

Exclusion Criteria:

• repeated and extensive traumatic events
• had other acute or severe psychiatric disorders or symptoms that warranted designation as the primary disorder (ongoing substance dependence, untreated bipolar disorder, OCD, psychotic symptoms, severe depression)
• were actively abusing analgesic medications (including narcotics), alcohol or other drugs
• had great difficulty to harbour and handle strong emotions that could lead to emotional outbursts or self-harming behavior
• had health risks due to medical reasons
• had social or economic difficulties or lack of social support that hindered behavior change
• current severe suicidal ideation that warranted immediate intervention (indicated by the MINI and a score of 3 to item 9 of the Patient Health Questionnaire 9-item version (PHQ9))