Diarrhea is one of the most socially restrictive symptoms associated with irritable bowel syndrome. Also, chronic diarrhea carries the possibility of dehydration, which, in severe cases, may lead to a patient being hospitalized for intravenous fluid and electrolyte replenishment. In general, patients suffering from D-irritable bowel syndrome use antidiarrheal agents on an as-needed basis; only in cases of severe D-irritable bowel syndrome are antidiarrheals used on a consistent dosage schedule.
Mechanism Of Action
Antidiarrheals reduce coloric transit, typically through direct action on the smooth muscle of the gastrointestinal tract. This action increases the bulk, density, and viscosity of the feces and prevents excessive loss of electrolytes and fluids. Most antidiarrheals prescribed for irritable bowel syndrome patients are active at opiate receptors in the gastrointestinal tract, which control smooth-muscle contractility.
Antidiarrheal agents are given orally, in either a tablet or syrup formulation.
Loperamide (McNeil’s Imodium, Janssen-Cilag’s Linguial, generics) is an orally active antidiarrheal available both OTC and by prescription. This agent is an antiperistaltic (it reduces contractility) that exerts an opioid-like effect by activating opioid receptors on circular and longitudinal muscles of the intestines.
This action reduces smooth-muscle contractions and increases transit time. Loperamide‘s secondary mechanisms of action enhance intestinal water and ion absorption, probably as a result of slower transit time, and strengthen rectal sphincter tone. The agent does not produce opiate dependency because it cannot cross the blood-brain barrier.
A randomized, double-blind study compared loperamide with placebo in the treatment of irritable bowel syndrome. Ninety patients received standard dosing of loperamide or placebo for five weeks. During the course of the study, the loperamide group experienced an improved stool consistency (32%), reduced defecation (36%), and reduced overall intensity of pain, although some patients in the loperamide group reported an increase in abdominal pain during the night.
A meta-analysis of four clinical trials of irritable bowel syndrome patients found that loperamide reduces diarrhea symptoms, including a statistically significant improvement in stool frequency and consistency. Clinical studies of loperamide have failed to demonstrate its efficacy in improving fecal urgency or abdominal distension.
Loperamide’s side effects are infrequent and mild; however, treatment may exacerbate symptoms of constipation in patients who have M-irritable bowel syndrome. Use of a liquid formulation of loperamide may reduce the potential for developing constipation.
The antidiarrheal diphenoxylate (Pfizer’s Lomotil, Sanofi-Synfhelabo’s Diarsed, generics) is available in tablet form by prescription in combination with atropine (generic) in the United States and Europe. Diphenoxylate reduces intestinal motility by activating the mu-opioid receptor in colonic smooth muscle.
Its inhibitory effect on intestinal smooth muscle reduces propulsion and peristaltic action and increases transit time. Because diphenoxylate can cross the blood-brain barrier, it has significant opioid activity, and higher doses can cause dependence. Atropine is added to diphenoxylate in the prescription formulation to discourage opiate abuse and relax stomach spasms.
Clinical data regarding the efficacy of diphenoxylate atropine in patients with irritable bowel syndrome are not available. However, diphenoxylate must be used with extreme caution in patients with liver disease; in such cases, it can cause hepatic coma. Atropine’s side effects, which are similar to those of cholinergic agents, include dizziness, insomnia, headache, glaucoma, and tachycardia; the latter two side effects limit use of this combination in elderly patients.