Zsa Zsa Weerts

Cost-effectiveness of peppermint oil in IBS 207 8 Supplementary Methods Patient inclusion Patients had to be between 18 and 75 years of age and needed to fulfill the Rome IV diagnostic criteria for IBS. If alarm symptoms were present ( e.g. unexplained rectal blood loss or weight loss), a colonoscopy or other relevant tests were performed to exclude organic disease. Exclusion criteria were inability to read or understand Dutch, history of GI disorders such as inflammatory bowel disease, celiac disease, or thyroid dysfunction (if not well-regulated), history of major abdominal surgery or radiotherapy interfering with GI function. An uncomplicated appendectomy, cholecystectomy, or hysterectomy were allowed unless within six months prior to screening. Other exclusion criteria were use of peppermint oil capsules in the three months prior to screening, a known allergic reaction to peppermint oil, current drug abuse, and a history of liver or gallbladder/biliary disease. Women had to use contraceptives and have a negative urine pregnancy test, or be postmenopausal for at least two years. The use of one antidepressant or one PPI was allowed, if a patient had been and would stay on a stable dose. Prohibited concomitant medications included opioids, prokinetics, stimulant laxatives ( i.e . bisacodyl), linaclotide, prucalopride, and anti-spasmodic drugs. Regular use of NSAIDs, antibiotics, osmotic laxatives, and antidiarrheal drugs was prohibited. Determination of indirect costs For the determination of absenteeism costs, self-reported dates of sick leave were used to determine the period for which absenteeism needed to be valued. Costs were calculated by multiplying the hours absent by average wage rates per hour (reference average wage rates used in this study can be found in Table 8.1 ). For the determination of presenteeism costs, productivity losses were valued by multiplying hours in which work was impaired with average wage rates per hour. The number of hours in which work productivity was impaired, was determined using a self- reported inefficiency score. Productivity losses in unpaid work were valued by multiplying the hours lost in productivity with the average wage for domestic help ( Table 8.1, main manuscript ), as recommended by the Dutch costing manual. 13 For all other indirect costs, we used general average wage rates per hour instead of job-specific wages, as recommended by the Dutch costing manual, as this leads to less bias in favor of high-income earners. Because of general disparities in wages as a result of gender, different averages were used for women versus men ( Table 8.1, main manuscript ).