The Influence of Tramadol on Opioid-induced Bowel Dysfunction

Inclusion Criteria: * Healthy (assessed by a study-affiliated medical doctor.) * Signed informed consent. * Able to read and understand Danish. * Male (to avoid influence of menstrual cycles) * Northern European descent (to minimize genetic variance influences on drug metabolism). * The researcher believes that the participant understands the study details, is compliant and is expected to complete the study. * Opioid naïve\* (who have not taken prescription opioid for one week or longer and at least a year have passed since completion of a previous opioid study.) * Between 20 and 40 years of age. * A State Trait Anxiety Inventory (STAI) score in the range of 20-37 i.e., classified as "no or low anxiety" at inclusion. Exclusion Criteria: * Known hypersensitivity or allergy towards the used pharmaceutical compounds or pharmaceutical compounds similar to those used in the study. * Less than three spontaneous bowel movements per week. * Participation in other studies within 14 days prior to first visit. * Expected need of medical/surgical treatment during the study. * Any diagnosed disease, which investigator concludes will affect the trial (including all contraindicated complications: severe chronic obstructive pulmonary disease, pulmonary heart disease, severe bronchial asthma, paralytic ileus, hypercapnia, serious respiratory depression with hypoxia, moderate to severe decreased liver function, gastrointestinal obstruction or perforation, acute surgical abdominal complications such as appendicitis, Mb. Crohn's, ulcerative colitis, and toxic mega colon). * History of substance abuse (alcohol, tetrahydrocannabinol (THC), benzodiazepine, central stimulants and/or opioids, urine drug test will be performed prior to treatment start). ¹ * History of major mental disorders (e.g., major anxiety, major depression or treatment with psychoactive medications etc.) ² * Metal implants or pacemaker. * Daily use of prescription only medicine * Daily alcohol consumption * Participation motivated by "wrongful" reasons such as poor economy, or psychosocial issues e.g., problems in the family, loneliness, sadness. People with such problems may be more likely to develop substance dependence.1 * Intake of alcohol within 48 hours before start of study period or any alcohol consumption during each study period. If consumption takes place during the study the participant will be excluded. * Use of any analgesic medication within 48 hours before start as well as for the duration of the study. If consumption takes place during the study the participant will be excluded. * Nicotine use