One-hundred and six (106) Subjects will be randomized to receive either dietary supplementation with L. reuteri DSM 17938 (2 chewable tablets with breakfast), or the same number of comparable placebo tablets, for 4 weeks. Supplementation will be stopped after 4 weeks, and the subjects will be followed up for an additional 4 weeks. The subjects will complete a diary to record frequency and intensity of pain, use of medication, deviation from the usual diet, physical activities, and absence from school and other activities. Gastrointestinal symptoms are assessed by the GSRS (Gastrointestinal Symptom Rating Score).