Effect of Methylnaltrexone on GI Transit in Healthy Volunteers

This is a single-center, randomized, double blind, placebo-controlled study evaluating the effects of placebo, codeine, methylnaltrexone and codeine with methylnaltrexone on gastrointestinal motility and colonic transit of solids in healthy human subjects. The hypotheses are: 1. Methylnaltrexone administered subcutaneously enhances gastrointestinal motility with acceleration of overall colonic transit, and ascending colon emptying of solids in healthy humans. 2. Methylnaltrexone significantly accelerates colonic transit that is delayed by codeine