Background/Aims: It has been suggested that STW5 (Iberogast) reduces heartburn symptoms in patients with functional dyspepsia, but underlying mechanisms of action are unclear. The aim of this study is to investigate whether STW5 affects esophageal sensitivity or esophageal motility, thereby reducing occurrence and perception of reflux events. Methods: We performed a double-blind, randomized, placebo-controlled, crossover trial in patients with functional dyspepsia (Rome IV) and reflux symptoms. After 4 weeks of treatment with either placebo or STW5, patients were studied with an esophageal acid perfusion test and ambulatory 24-hour pH-impedance monitoring. Results: A total of 18 patients (7 men, median age 54, range [19-76]), were included in the study. Although we found no statistical difference in our primary outcome the total Reflux Disease Questionnaire score 2.33 (0.25-4.33) vs 2.67 (1.17-4.00), P = 0.347, “gastroesophageal reflux disease” and “regurgitation” subscale scores were lower after STW5 treatment compared to placebo (P = 0.049 and P = 0.007). There was no statistical difference in number of reflux events, acid exposure time and acid sensitivity scores between STW5 and placebo. In a subgroup analysis of patients with pH-metry confirmed gastroesophageal reflux disease, treatment with STW5 significantly reduced the total number of acidic reflux events (P = 0.028). Moreover, in patients with reflux esophagitis, the median lag time to acid perception increased after STW5 treatment (P = 0.042). Conclusions We found some indications pointing towards a beneficial effect of STW5 on reflux symptoms in dyspeptic patients, with reduction of esophageal hypersensitivity as a potential underlying mechanism. Our findings will have to be confirmed in larger studies.