The goal of this randomized controlled trial is to investigate if online transdiagnostic cognitive behavior therapy (CBT) can be effective in the treatment of common mental disorders in adult primary care patients. The main aim is to investigate:

if online transdiagnostic CBT yields superior symptomatic improvement compared to care-as-usual when given in a primary care context to patients with a common mental disorder.

Researchers will compare online transdiagnostic CBT to primary care as usual to see if psychiatric symptoms can be reduced.

Participants will be randomized to online transdiagnostic CBT or primary care as usual. Participants in online transdiagnostic CBT will receive a cognitive behavioral treatment provided through a secure web platform in which psychoeducation, worksheets, and exercises are presented in online modules similar to chapters in a book. Participants will receive weekly written feedback on their work and progress by a therapist who gives gradual access to the treatment modules. The lion's share of the contact between the therapist and the patient thus takes place through messages (similar to email) in the online treatment platform.

Participants randomized to care as usual will receive health care services as usual in primary care. As this comparator is intended to reflect real routine primary care, the interventions will not be controlled by the research project, but the investigators expect that participants will receive an active treatment.

Background: Common mental disorders (anxiety, depression and stress-related disorders) are a main driver of disease burden and primary care is the first-line of care. There are substantial disadvantages and barriers to implementing single-disorder psychological treatment leading to low treatment accessibility. These problems can potentially be overcome by the use of online transdiagnostic CBT, which draws on shared underlying mechanisms and is resource-efficient. There is still clear limitations in its evidence-base and online transdiagnostic CBT needs to be further investigated in randomized controlled trials in a primary care context.

Purpose and aims: The overall purpose of the current research project is to build evidence for an accessible online transdiagnostic CBT for primary care patients with common mental disorders. A randomized controlled trial will be conducted where adult primary care patients with a common mental disorder will receive online transdiagnostic CBT or care as usual.

The main aim (I) of the study is to investigate if Internet-delivered transdiagnostic CBT yields superior symptomatic improvement compared to care-as-usual when given in a primary care context to patients with a manifest common mental disorder. Secondary aims are to investigate (II) if the treatment conditions are associated with improvement in quality of life, functional impairment, and neuroticism (III) moderators of treatment outcome, (IV) mediators of improvement, and (V) the cost-effectiveness (including effects on sickness absence) of online transdiagnostic CBT compared to primary care-as-usual. Should the trial fail to show a significant difference on the primary outcome, a secondary aim will also be to (VI) investigate if online transdiagnostic CBT is non-inferior to primary care as usual in reducing psychiatric symptoms.

Methods: This is a randomized controlled superiority trials where consecutively recruited adult primary care patients (N=500) are allocated in a 1:1 ratio to Internet-delivered transdiagnostic CBT or to primary care-as usual. This trial is part of larger project comprising two twin randomized controlled trials that are conducted in parallel where the difference between them is that the current trial will include patients with a manifest common mental disorder whereas the other (separately registered with clinicaltrials.gov) will include patients with subsyndromal complaints.

Participants are consecutively recruited from the regular influx of primary care patients. Gustavsberg Primary Health Care Center in Region Stockholm, Sweden, is the basis, but additional primary care centers will be engaged to facilitate the recruitment and treatment of participants. The investigators expect that 10 to 20 primary care centers in Region Stockholm will be engaged for the recruitment and treatment of participants.

Patients who seek help for common mental health problems will be asked if they are interested in applying for the study. Those interested will undergo an assessment interview with a licensed clinician where all inclusion criteria are checked.

Measurements: See section Outcome Measures.

Treatment conditions: See section Arms and Interventions.

Data analysis Change in the primary outcome measure will be analyzed using mixed effects linear regression. Fixed predictors in these analyses will be time, group and their interaction effect while taking individual variation in baseline symptom levels and change over time into account, i.e., random intercept and slope. Change from baseline to 10-week follow-up will be the primary endpoint. Power analyses show that to have 90% power to detect an effect size of d=0.25 (α=.05), given a correlation between measurements of 0.7, and an expected attrition of 15-20%, 250 participants will be needed in each arm (total sample size N=500). Should the main analysis show that there is a non-significant difference on the primary outcome, the investigators will conduct a secondary analysis of whether online transdiagnostic CBT is non-inferior to primary care-as-usual where the non-inferiority margin on the primary outcome is set to d=0.25, i.e., the bound of the one-sided 95% confidence interval must be within this margin for non-inferiority to be demonstrated. With 500 participants, 15-20% attrition, and a true null effect of zero, the study will have approximately 80% power to detect non-inferiority.

Inclusion Criteria:

To be included patients have to:

1. have a common mental disorder in form of an anxiety disorder (social phobia, panic disorder, agoraphobia, general anxiety disorder, or specific phobia), health anxiety (hypochondriasis or illness anxiety disorder), obsessive-compulsive disorder, depression (depression or dysthymia), or a stress-related disorder (exhaustion disorder, adjustment disorder, or post-traumatic stress disorder), that warrants treatment,
2. be at least 18 years old,
3. have regular access to a device with an Internet connection,
4. be able to read and write in Swedish,
5. have no severe psychiatric disorder that requires specialized psychiatric care, such as psychosis, bipolar disorder, or anorexia nervosa,
6. if on medication with monoamine agonist, have stable dosage in the past month,
7. have no on-going psychological treatment, and
8. provide written informed consent for participation and complete baseline assessment

The information collected in the study contains sensitive personal data and is protected by Swedish and European data secrecy regulations. Any data sharing is thus subject to assessment by relevant authorities and is not something the principal investigator can decide upon.