A medical trial is being held to see if a drug developed as an anti-depressant could become a common treatment for Irritable Bowel Syndrome (IBS).
Amitriptyline has sometimes been prescribed to people with IBS when other treatments have been ineffective.
But experts said they needed to know more about why the drug helps before it can become more widely used.
A group of 500 volunteers is now being recruited from GP surgeries around Bristol, Leeds and Southampton.
According to charity the IBS Network, the condition affects about 12 million people in the UK.
There is no cure and patients can experience abdominal pain, bloating and constipation, among other symptoms.
Experts at the University of Bristol are leading the medical trial, which is called Atlantis.
The 500 test patients will be given either