And the problems can last for months, even years, without relief.
Irritable bowel syndrome
The condition is called irritable bowel syndrome, or IBS, a functional disorder of the intestines. And while it can be very debilitating, the good news is that IBS doesn't cause permanent harm to the intestines and won't lead to intestinal bleeding or life-threatening diseases like cancer.
What IBS does cause are spasms in the colon, a six-foot-long tube that connects the lower intestines to the rectum. It's the place where waste matter collects and forms into stool, and movements of the colon help push the waste along the pathway until it's ready to exit the body, says gastroenterologist Dr Steven Field.
"Most of the pain and symptoms of IBS occur when sections of the colon are experiencing these spasms," says Field, an assistant clinical professor of medicine at New York University Medical Centre.
The spasms are also responsible for keeping stool from reaching the rectum (causing constipation), or pushing it there too quickly (causing diarrhoea), Field adds.
Finding a cause
Although IBS was once thought to be caused by emotional upsets - a kind of super version of a "nervous stomach" - this is no longer believed to be the case. While emotions can make IBS symptoms worse, Field says, most doctors believe there's a physiological cause behind this illness.
No one is certain exactly what that cause is, but one of the latest theories ties symptoms to levels of serotonin, a biochemical traditionally associated with depression.
"What most people don't realise is that there are not only receptors for serotonin in the brain, which is the link to depression, there are also receptor sites in the gut. And there is some research to show they may play a role in IBS," Field says.
Not surprisingly, one of the more recent drug treatments for irritable bowel syndrome involves SSRIs, selective serotonin reuptake inhibitors, gastroenterologist Dr Mark Pochapin says. These are medications you may recognise as "anti-depressants" -- drugs like fluoxetine (Prozac) and paroxetine (Paxil, Aropax).
"For many people, they have proven somewhat effective," says Pochapin, an assistant professor of medicine at New York-Presbyterian Weill Cornell Medical Centre in New York.
Although the receptors in the gut are slightly different from those in the brain, Field says the antidepressant drugs do have some crossover-effects that enable them to ease IBS symptoms.
In the past, antidepressants known as tricyclics were also found to be helpful to many people with IBS. But experts say SSRIs work in an entirely different way and may be able to help those who did not find relief on the older medications.
New drug treatment
Among the very newest drug treatment approaches, says Pochapin, are IBS medications known as "antagonists," drugs that work to inhibit the activity of a chemical in the body, or "agonists," drugs that combine with a receptor on a cell to initiate a new or different chemical activity.
One of the first examples of antagonists, a medication known as Lotronex, was taken off the American market in late 2000 after numerous reports of serious constipation-related complications, and several deaths.
Despite its problems, doctors report that many IBS sufferers, particularly those whose main symptoms were diarrhoea, found Lotronex to be very beneficial.
In addition, a second antagonist drug known as cilansetron is now in clinical testing and may be available in the US as soon as 2003. Doctors say it provides all the benefits of Lotronex, with fewer side effects.
Another promising antagonist is known as Zelnorm/Zelmac. It works best when constipation and abdominal pain are the primary symptoms. It has been used successfully in Mexico, Switzerland, Australia and several other countries.
In addition to medication, Field says high-fibre diets and plenty of exercise can help against IBS, along with a reduction in foods that cause bloating and gas, like dairy products or red meat. And small, frequent meals may be easier to tolerate than larger meals consumed less frequently.
"The point to remember is that there is help available if you have IBS. And there are many medications, as well as treatment strategies, that can help reduce the symptoms and help patients cope," Field says.