This double-blind, randomized, multicenter, prospective clinical trial evaluated a commercial formulation of charcoal tablets (Eucarbon) and tablets containing only nonactivated charcoal (carbo ligni [CL]) in 284 patients between the ages of 19 and 70 years with irritable bowel syndrome (IBS). After 12 weeks, 262 patients were available for intention-to-treat analysis. Overall well-being, the primary efficacy parameter, was determined by means of the Francis scoring system. Eucarbon treatment alleviated symptoms by about 60%, but the relative gain in efficacy vis-à-vis the CL group was only 9%. Several clinical observations and subgroup analyses, however (eg, in patients suffering from constipation), showed that Eucarbon was more effective. Both treatments were well tolerated and produced adverse events at a similar frequency (22%, Eucarbon vs 17%, CL). In most cases, it was not possible to distinguish the adverse event from symptoms of IBS. The herbal preparation Eucarbon was effective and safe in IBS, a complex disease requiring long-term treatment.