Irritable bowel syndrome (IBS) is a functional gastrointestinal disease. There is no well-defined pharmacological treatment. This clinical trial is a prospective, double-blind, two-armed randomized controlled, single-center trial. It is created to examine the role of IBS in patients with lactose intolerance. IBS patients undergo lactose H2 breath test (LHBT) and lactose tolerance test (LTT). Those with positive LTT and LHBT will be randomized into two groups: alverine-citrate + simethicone and lactase group (1) or alverin-citrate + simethicone with the placebo group (2). The goal of this study is to compare the lactase enzyme with placebo in IBS patients with lactose intolerance.