Irritable bowel syndrome (IBS) is both a physiologic and psychological syndrome. Thus, pharmacological treatment may be geared towards alleviating gastrointestinal symptoms; concurrently, medications may be prescribed to temper the pain component of IBS or modify underlying affective disorders and psychosocial factors that may be contributing to or resulting from symptom severity. The multifactorial pathophysiology of irritable bowel syndrome (IBS), the waxing and waning of both colonic and noncolonic symptoms, and inordinate placebo effect noted in this population make it difficult to develop a therapeutic plan and evaluate its efficacy. The following sections discuss classes of agents used to treat symptoms of IBS, including novel therapies and agents.
Dietary Fiber and Bulking Agents
Dietary supplements such as wheat bran or psyllium are most commonly used for constipation. In IBS patients, these agents have been studied in the treatment of diarrhea. Patients given 6.4 g of psyllium per day for an eight-week period had some improvement in the number of normal stools, stool consistency, and pain frequency as compared to baseline, as well as a mild decrease in pain severity and gas. The placebo group achieved statistically significant improvement in these same parameters. Several studies with wheat bran in doses of up to 30 g per day versus placebo resulted either in significantly improved abdominal pain and bowel habits in both groups or a higher rate of improvement in the placebo group. In these studies, there was a tendency towards significant increases in stool weight that correlated with the amount of fiber ingested but not with improvement or worsening of symptoms. A similar scenario was noted in another study using corn bran (20 g per day), in that although many symptoms improved significantly from baseline during the study in the active treatment group, the same was true for the placebo group.
Psyllium (ispaghula husk) for patients with either constipation-dominant or alternating constipation/diarrhea irritable bowel syndrome (IBS) resulted in a significant decrease in mean transit time and fewer days of absent bowel movement as compared to placebo, with the effect on abdominal pain and distension being similar to that for placebo. Nondigestible bulking agents such as polycarbocil calcium may also help diminish constipation, bloating, and abdominal pain.
In general, bulking agents are not considered an effective treatment when compared to placebo. Response to and adverse effects of these agents (flatulence, constipation, worsening diarrhea) vary depending on the type and amount of fiber ingested. However, considering the placebo response noted in the studies, a trial period with these products may prove beneficial in some patients.