Effect of phenolphthalein on the function and structure of rodent and human intestine.

Phenolphthalein (PHE) is the active ingredient in at least 12 over-the-counter laxative preparations. It is alleged to have a minor effect in the small intestine compared with the colon where it is supposed to alter motor activity or water absorption or both. We measured water absorption from segments of rat intestine in vivo, and from the nearly intact small intestines of patients with established ileostomies. In rats, each intestinal segment served as its own control so that water absorption in a test period (PHE, 1--15 mg/100 ml) could be expressed as a percentage of the control period when buffer was infused. PHE inhibited water absorption, and the degree of inhibition was related linearly to log10 PHE concentration. The similarity of slopes of the regression lines suggested that all segments of the rat intestine were equally sensitive to PHE. In six patients, PHE, 100 mg four times a day, increased weight of ileostomy output by 30%, and Na+ output by 39%. These facts indicate that some of the laxative effects of PHE result from inhibition of water absorption in both large and small intestines.