Iberogast is a complex herbal preparation. As a fixed drug combination (9 constituents) it is composed of a fresh plant extract of Iberis amara and of extracts of 8 other dried herbal drugs ( Chelidonii herba, Cardui mariae fructus, Melissae folium, Carvi fructus, Liquiritiae radix, Angelicae radix, Matricariae flos, Menthae piperitae folium). The pharmacological effects as well as the therapeutic effectiveness, tolerability, and toxicity of Iberogast were experimentally and clinically recorded and documented using modern investigation tools. Both the experimental as well as the clinical studies indicated a regulatory influence of Iberogast on the whole gastrointestinal tract by a special dual action. While the included extracts of the dried herbal drugs have mainly spasmolytic properties, the fresh plant extract of Iberis amara has a tonic effect on the gastrointestinal tract. Depending on the predistension of the gastric or intestinal wall, the tonic or the spasmolytic effects of Iberogast prevail. Both the fresh plant extract of Iberis amara and the combined preparation of Iberogast were found to be toxicologically safe in therapeutically effective doses. For the estimation of the clinical effectiveness a systematic review was performed (data research: January 1970 to September 2002). As shown in controlled (according GCP standard) as well as supportive and uncontrolled clinical studies, the symptoms of functional dyspepsia and of irritable bowel syndrome (one controlled study and one observational study) could be significantly reduced by these herbal preparation in comparison to placebo. Two trials comparing Iberogast with the prokinetics metoclopramide and cisapride demonstrated a comparable therapeutic effectiveness of the herbal preparation and the prokinetics in the treatment of dyspepsia. Adverse events were rare and, with respect to frequency and spectrum, partly the same as found with placebo. Another advantage of Iberogast is that it targets only the gastrointestinal tract and the enteral nervous system, but not the central nervous system. Because of its special dual action, its clinically proven effectiveness, and its good tolerability, Iberogast may be a drug of first choice in the treatment of functional gastrointestinal diseases and their corresponding symptoms.