the primary goal of this study was to compare gastrointestinal symptom reduction in patients on bismuth-containing quadruple eradication therapy supplemented with Lactobacillus reuteri strains (DSM 17938 and ATCC PTA 6475) or placebo. this was a randomized, double-blind, parallel-arm, placebo-controlled clinical trial. Patients received a first-line eradication regimen based on bismuth subcitrate potassium, metronidazole, tetracycline hydrochloride (three-in-one capsules) and omeprazole 40 mg twice a day for ten days, plus a probiotic or placebo tablet for 30 days. During follow-up, gastrointestinal symptoms were assessed using an evaluation scale (GSRS), and adverse events were collected at 0, 14, 28 and 56 days. a total of 80 patients were included from February 2018 to May 2019 at a single site. Eradication therapy was effective in 85 % of patients, with no differences between treatment arms. In the group receiving the probiotic, abdominal pain decreased in 42 % of patients, compared with 19 % in the control group (OR: 0.27; CI, 0.13-0.58; p < 0.001), and abdominal distension decreased in 25 % versus 17 % in the control group (OR: 0.24; IC, 0.19-0.84; p < 0.001); Conclusions: treatment with L. reuteri only reduced abdominal pain and distension. Further studies are needed to establish the role of probiotics as adjuvant therapy in H. pylori eradication.