Peppermint Oil for the Treatment of Irritable Bowel Syndrome: Optimizing Therapeutic Strategies Using Targeted Delivery

Inclusion Criteria: 1. Age between 18 and 75 years; 2. Diagnosed with Irritable Bowel Syndrome according to the Rome IV criteria: (Recurrent abdominal pain, at least 1 day/week for the last 3 months; Symptom onset at least 6 months prior to diagnosis; 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: 1. Inflammatory Bowel Disease; 2. Celiac Disease; 3. 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. 5. Average worst abdominal pain score (on 11-point NRS) of \> 3, during the two-week run-in period. Exclusion Criteria: 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; 1. The use of 1 antidepressant drug is allowed, providing dosing has been stable for \> 6 weeks before enrollment; 2. 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: 1. Uncomplicated appendectomy, cholecystectomy and hysterectomy allowed unless within the past 6 months; 2. Other surgery upon judgment of the principle investigator; 5. History of liver disease, cholangitis, achlorhydria, gallstones or other diseases of the gallbladder/biliary system; 6. Pregnancy, lactation; 7. Using drugs of abuse; 8. Known allergic reaction to peppermint.