Patient Education and Duloxetine, Alone and in Combination, for Patients With Multisystem Functional Somatic Disorder

The goal of this clinical trial is to test if patient education or duloxetine can be used to treat multisystem functional somatic disorder (FSD). The main questions it aims to answer are: * Does duloxetine work better than placebo in the treatment of FSD? * Does patient education work better than usual treatment for FSD? * Does the combination of patient education and duloxetine work better than using only one of these treatments? Participants are patients with FSD. They will receive one of six different treatment combinations: 1. Patient education alone (three individual consultations with a doctor and one group session) 2. Treatment as usual (receiving the diagnosis and a short presentation of what FSD is) 3. Duloxetine 4. Active placebo (a treatment that looks like duloxetine and has similar side effects, but no known effect on FSD) 5. Patient education and duloxetine 6. Patient education and active placebo Researchers will compare the groups receiving patient education with those receiving only treatment as usual to see if patient education is a better treatment than just receiving a diagnosis and short explanation. Furthermore, researchers will compare the groups receiving duloxetine to those receiving placebo to see if duloxetine works better than placebo. Finally, researchers will compare the groups receiving both patient education and duloxetine to those receiving only one of these treatments to see if the combination works better than the treatments given alone. The researchers will also collect samples of blood and stool in a biobank to be used in future research.