This will be a double-blind, placebo-controlled, randomized, parallel group study to assess the safety and efficacy of CHT. 100 subjects with ROME II criteria IBS will provide informed consent and undergo screening procedures. Subjects fulfilling eligibility requirements will be randomized to one of the two treatment arms (will receive a subject number). After a two-week baseline observation period, and reconfirmed eligibility, subjects will be allotted enough study drug for the self administration of a daily dose of 25 mg CHT or placebo, for 4 weeks. The Week 4 clinic visit will be followed by a four week follow-up period without treatment. The day of randomization will be the week -2 visit and start of study treatment will be at the Week 0 study visit.