A Study to Explore the Influence of Two Opioid Pain Medications on Bacterial Composition in the Gut and Other Gastrointestinal Aspects

Inclusion Criteria: Subjects who are receiving WHO step II/III opioid analgesic medication for the treatment of non-malignant pain and who require daily opioid treatment for pain with WHO step III opioid therapy for the duration of the study, based on Investigator's judgement. Documented history of non-malignant pain that requires around-the-clock opioid therapy (20 - 50 mg oxycodone PR equivalent per day for a minimum of study duration). Subjects with constipation caused or aggravated by opioids: * Subject's medical need of regular intake of laxatives to have at least 3 bowel evacuations per week, or having less than 3 bowel evacuations when not taking a laxative. * In the opinion of the subject and investigator confirm that the subject's constipation is induced, or worsened by the subject's prestudy opioid medication (present at Screening). Exclusion Criteria: Any contraindication to oxycodone, naloxone, or any non-investigational medicinal products (NIMPs) that will be used by subjects during the study. Continuous systemic use of antibiotics and/or steroids within the last 4 weeks prior to the start of the Screening Period and during the study period. Chronic or intermittent pain that results from Fibromyalgia or Rheumatoid Arthritis