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Cost-effectiveness of peppermint oil in IBS 193 8 Health-related resource use was assessed using the iMTA Medical Consumption Questionnaire (MCQ), which is designed to measure costs in the Dutch healthcare system (see https://www.imta.nl ). The MCQ was completed at baseline and after eight-weeks of treatment. As the recall-period of the MCQ is three months, the recall period was adjusted to eight weeks for assessment at the end of the eight-week treatment period. After all data had been collected, a distinction was made between IBS-unrelated and potentially related costs based on expert opinion; e.g. drug treatment for comorbid cardiovascular disease was not included, whereas visits to a gastroenterologist or GP, or mental healthcare were. Direct costs were calculated by multiplying resource use by the cost price per resource unit, adopting reference prices derived from the Dutch costing manual ( Table 8.1 ). Medication costs were obtained from the Dutch Pharmacotherapeutic Compass (Healthcare Insurance Board, 2017). Additionally, for each visit to a care provider, travel expenses were calculated using a standard cost of €0.19 per average kilometer. Indirect health-related costs were measured using the iMTA Productivity Cost Questionnaire (PCQ), which was designed and validated for the Dutch situation (see https://www.imta.nl) . 15 The PCQ was completed at baseline and at four and eight- weeks of treatment. The PCQ includes questions on productivity loss of paid work (absenteeism and presenteeism) and productivity loss of unpaid work ( e.g. voluntary work, or homemaking and caregiving). Indirect costs were calculated by multiplying the hours absent (using self-reported dates of sick leave) or impaired (using a self-reported inefficiency score) by average wage rates per hour. Since the majority of costs and outcomes were measured within one year, no discounting was applied and all costs were indexed for inflation to 2017. A detailed description of the calculation of indirect costs is given in the Supplementary Material. Table 8.1 Overview of costs per unit of resource use. Resource use Unit Cost (euro) Reference Study treatment Placebo Small-intestinal release peppermint oil 168 capsules 168 capsules 0.00 41.86 Manufacturer Manufacturer General practitioner consultation Gastroenterologist consultation consultation/visit consultation/visit 34.00 93.00 Dutch costing manual Dutch costing manual Social work consultation Mental healthcare consultation consultation/visit consultation/visit 67.00 94.60 Dutch costing manual Dutch costing manual Travel cost car or public transport Parking cost kilometer visit 0.19 3.07 Dutch costing manual Dutch costing manual Average wage women Average wage men Productivity cost unpaid work hour hour hour 32.36 38.82 14.34 Dutch costing manual Dutch costing manual Dutch costing manual