Ancestral medicinal use of guava (Psidium guajava L. Fam. Myrtaceae) is today supported by numerous biomedical studies concerning the properties of leaf extracts. However, insufficient clinical studies are reported on the use of this plant resource in the treatment of gastrointestinal ailments. The present work reports a randomized, double-blinded, clinical study performed to evaluate the safety and efficacy of a phytodrug (QG-5) developed from guava leaves, standardized in its content of quercetin and orally administered to a group of adult patients with acute diarrheic disease. Capsules containing 500 mg of the product were administered to 50 patients every 8 h during 3 days. Results obtained showed that the used guava product decreased the duration of abdominal pain in these patients.