Comparison of the Effects of Tapentadol and Oxycodone on Gastrointestinal and Colonic Transit in Humans

Healthy volunteers Inclusion criteria 1. Males and non-pregnant, non-breastfeeding females 2. 18-65 years old Exclusion criteria 1. Use of any mu-opioid agent in the last 3 months 2. Structural or metabolic diseases/conditions that affect the gastrointestinal system, or functional gastrointestinal disorders. For screening the shortened screening version of the Bowel Disease Questionnaire (Appendix) will be used to exclude subjects with dyspepsia, irritable bowel syndrome or significant gastrointestinal symptoms. Of 19 questions, participants have to have three or less positives to be eligible to participate. 3. Unable to withdraw medications 48 hours prior to the study : * Alter GI transit including laxatives, magnesium or aluminum-containing antacids, prokinetics, erythromycin, narcotics, anticholinergics, tricyclic antidepressants, selective serotonin re-uptake inhibitors (SSRIs) and newer antidepressants. * Analgesic drugs including opiates, nonsteroidal anti-inflammatory drugs (NSAIDs), COX 2 inhibitors * SSRI NOTE: Low stable doses of thyroid replacement, estrogen replacement, low dose aspirin for cardioprotection and birth control pills or depot injections are permissible. 4. Female subjects who are pregnant or breast feeding. 5. Clinical evidence (including physical exam, ECG, hemoglobin level and review of the medical history) of significant cardiovascular, respiratory, renal, hepatic, gastrointestinal, hematological, neurological, psychiatric, or other disease that interfere with the objectives of the study. 6. Subjects who are considered by the investigator to be alcoholics not in remission or known substance abusers. 7. Subjects who have participated in another clinical study within the past 30 days 8. History of porphyria, renal (creatinine \> 1.5mg/dL) or significant liver impairment (transaminases, alkaline phosphatase of gamma-glutamyl transpeptidase (GGT) \>2 times upper limit of normal)