Irritable bowel syndrome is a common condition affecting 1 in 10 of the population. About a third of these suffer from diarrhoea, which severely impairs their quality of life. Previous studies in Nottingham have suggested that some patients with diarrhoea may have an excess of a chemical called serotonin in their gut. Serotonin stimulates secretion and propulsion in the gut and contributes to diarrhoea. We are interested to see whether a drug, Ondansetron, which blocks the effect of serotonin, would improve symptoms in patients with IBS and diarrhoea. We think the drug may work better in people with a specific gene type so your genetic makeup may be of influence and we would like to test this. Because IBS symptoms fluctuate, one way to determine whether Ondansetron is effective is to perform a randomised placebo controlled trial in which neither the patient nor the doctor knows which medication is being taken in each part of the study.

Inclusion Criteria:

• IBS-D patients meeting the Rome III criteria.
• Male or female aged 18-75 years
• Women of child bearing potential (who have a negative pregnancy test) must agree to use methods of medically acceptable forms of contraception during the study., (e.g. implants, injectables, combined oral contraceptives, sexual abstinence or vasectomised partners)
• Patients who are able to give informed consent.

Exclusion Criteria:

• Women who are pregnant or breastfeeding
• Patients that, in the opinion of the investigator, are considered unsuitable.
• Patients who have had abdominal surgery which may cause bowel symptoms similar to IBS (Please note, appendicectomy and cholecystectomy is not an exclusion)
• Patient unable to stop anti-diarrhoeal drugs
• Patients currently participating in another clinical trial or who have been in a trial in the previous three months

Since many patients will be on SSRIs or tricyclics antidepressants these will not be an exclusion criteria, provided they have been on medication at least 3 months and that the dose remains unaltered throughout the study.