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Chapter 9 232 Supplementary Methods In- and exclusion criteria Eligibility criteria for pre-treatment period In order to be eligible to participate in the run-in period of this study, subjects must meet all the following criteria: 1. Age between 18 and 75 years; 2. Diagnosed with Irritable Bowel Syndrome according to the Rome IV criteria 26 : x Recurrent abdominal pain, at least 1day/week for the last 3 months; x Symptom onset at least 6 months prior to diagnosis x Associated with two or more of the following: 1. Pain related to defecation; 2. Pain associated with a change in frequency of stool; 3. Pain associated with a change in form (appearance/consistency) of stool; 3. Based on the medical history and previous examination, no other causes for the abdominal complaints can be defined. Especially no history of: a. Inflammatory Bowel Disease; b. Celiac Disease; c. Thyroid dysfunction (if not well-regulated); If alarm symptoms (including unexplained rectal blood loss or weight loss) are present, a colonoscopy has been performed and was negative for other causes. 4. Women in fertile age (<55 years old) must use contraception or be postmenopausal for at least two years. Exclusion criteria for pre-treatment period A potential subject who meets any of the following criteria will be excluded from participation in this study: 1. Insufficient fluency of the Dutch language; 2. Any previous use (also incidental use) of peppermint oil capsules in the last 3 months prior to inclusion (the use of peppermint tea, menthol candy etc. is allowed); 3. The inability to stop regular use of medication affecting the gastro-intestinal system (such as Non Steroidal Anti Inflammatory Drugs (NSAID), laxatives, prokinetics, opioids, smasmolytics and anti-diarrhoeal drugs). This use should be halted at least 1 week before enrollment into the run-in period; a. The use of 1 antidepressant drug is allowed, providing dosing has been stable for >6 weeks before enrollment; b. The use of 1 proton pump inhibitor (PPI) is allowed, providing dosing has been stable >6 weeks before enrollment; 4. Previous major abdominal surgery or radiotherapy interfering with gastrointestinal function: a. Uncomplicated appendectomy, cholecystectomy and hysterectomy allowed unless within the past 6 months;