This was a single center, randomized, double-blind, placebo-controlled, parallel-group, Phase I study of the effects of naloxegol, a novel mu-opioid antagonist, on gastrointestinal and colonic transit in the presence or absence of the mu-opiate, codeine. There is a need to develop effective medications for the treatment of opiate-induced constipation and other motility disorders. Currently available opiates are complicated by addictive potential and induction of troublesome constipation.