16.03.3 Opioid-Induced Constipation

The opioids e.g. morphine, are commonly used in the treatment of severe pain or pain associated with terminal disease. The analgesia is due to stimulation of opioid µ-receptors in the central nervous system (Figure 16.5). However stimulation of opioid µ-receptors on the gastrointestinal tract leads to constipation (Figure 16.5). Thus, constipation is a common side-effect of opioid use for pain relief or abuse for euphoria.

Figure 16.5 Interaction between morphine and methylnaltrexone (Copyright QUT, Sheila Doggrell)

Methylnaltrexone is an antagonist at opioid µ-receptors. It can be administered orally, subcutaneously or intravenously. Methylnaltrexone does not cross the blood-brain barrier. Methynaltrexone antagonises the ability of opioids to cause constipation (Figure 16.5) but, because it does not cross the blood-brain barrier. it does not antagonise the centrally-mediated analgesic effect of morphine. Methlynatrexone is used in combination with morphine to give pain relief (due to the morphine) without the constipation (morphine-induced constipation prevented by methylnaltrexone).