This is a phase IIB, single center, randomised, double-blind, placebo controlled study of the treatment of pain and discomfort associated with IBS in which study medication or placebo will be administered as required for the relief of these symptoms up to 2 tablets, 4 times per day. After a 2-week run-in period to characterize the nature, frequency and severity of pain to ensure eligibility for the study, participants will treat these pain episodes as the need arises but with strict recommendations on how to self-administer the medication (number, time interval etc?) over a period of 4 weeks. No other pain medications will be permitted during these first 6 weeks (2 weeks run-in, 4 weeks of randomized treatment). During the study, patients will collect daily data which will characterize their pain and other symptoms of IBS, and adequate relief of pain/discomfort. The participants will also keep track of their symptoms and any medications used during a two week observation period after the last dose of medication. A blood sample will be taken and DNA extracted to assess whether IBS patients have single nucleotide polymorphisms in the gene for the kappa receptor.