Overview of the study interventions. The four boxes illustrate the visits to the doctor, and the content in the boxes include the different outcome measures. At visit 1, the patients were randomly and double-blindly assigned to receive either doxycycline or a placebo treatment during 3 weeks. At the third visit, the patients received the opposite study drug, in a cross-over manner. The vertical black arrows indicate time points of blood sampling for analysis of cytokine responses in blood (at the start of, after 5 days, and at the end of treatment, respectively). During the six-week wash-out period, no assessments or examinations were carried out. Abbreviations: SSS, symptom severity score; SF-36, Short Form-36.