Objective: comparison of efficacy of Mentha pulegium L. with that of placebo in treatment of patients with functional dyspepsia. Study design: randomized, double-blind, placebo-controlled, two group parallel design, single-center, phase 2 of clinical trial. Study population: patients with functional dyspepsia attending Baqiyatallah Hospital (Tehran, Iran). Inclusion criteria: patients aged 20 to 80; patients with functional dyspepsia according to the ROME III criteria; patients in whom upper gastrointestinal endoscopy has not shown any organic causes of dyspepsia; patients without organic, systemic and metabolic diseases causing dyspepsia. Exclusion criteria: patients with peptic ulcer disease, inflammatory bowel disease, irritable bowel syndrome, pure gastro-esophageal reflux disease and any organic gastrointestinal disease; patients with a history of Helicobacter pylori infection eradicating drugs use within the past 3 months; patients with a history of gastrointestinal system surgery; patients with background systemic diseases such as diabetes mellitus, heart failure, hepatic failure, renal failure, asthma, chronic obstructive pulmonary disease, neoplasms and severe psychiatric diseases; patients addicted to alcohol and opium; patients using cardiac, antihypertensive, antipsychotic, antianxiety, antibiotic and corticosteroid drugs and iron and calcium; patients with a history of discontinuing prescribed pharmacotherapy and incomplete treatment; pregnant women; women planning pregnancy; breast-feeding women. Sample size: 100 patients. Interventions: pennyroyal leaf extract is administered to a group of about 50 patients at the dose of one 330 mg capsule every 8 hours for 2 months and placebo capsule is administered to another parallel group of about 50 patients every 8 hours for 2 months. Before intervention and 2 months after intervention, the primary and secondary outcome variables are evaluated. Primary outcome variable: dyspepsia. Secondary outcome variables: Helicobacter pylori infection, quality of life.