The study is intended to explore changes in the composition and quantity of gut bacteria subject to treatment with strong pain medication. Two pain medications will be compared (OXN PR and OxyPR). Other gastrointestinal parameters will be assessed.

Patients who require around-the-clock opioid therapy and show symptoms of constipation secondary to opioid treatment will be randomised to receive either OXN PR followed by OxyPR, or vice versa. Each treatment takes 24days. The study is composed of three phases, a pre-randomisation phase, a double-blind phase and a Follow-up phase.

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