For many patients taking opioids for pain relief one of the most distressing side effects is constipation. Naloxone is effective in the reversal of the effects of opioids and is used following opioid overdose. If naloxone is given by mouth it would relieve the effects of constipation but as it goes into the blood stream very quickly, it would also reverse the effects of the opioid and therefore stop the pain relief. The aim of this study is to examine a slow release formulation of naloxone to see if is can reduce constipation without reducing the pain relieving effects of the opioid.