Methylnaltrexone is a peripherally acting mu (μ) opioid-receptor antagonist, predominantly active in the gastrointestinal tract, marketed in Australia as Relistor®. It does not cross the blood-brain barrier, or interact significantly with the delta or kappa opioid receptors.
Methylnaltrexone is indicated for use in opioid-induced constipation in patients with advanced illness who are receiving palliative care, when response to laxative therapy has not been sufficient. It is available only as a subcutaneous injection to be given when the patient has not experienced a bowel motion for at least 3 days.
There is no doubt that methylnaltrexone is effective at reversing opioid-induced constipation. Rapid onset of a bowel movement may occur within 30 minutes of a dose, therefore patients should remain close to toileting facilities.
Download the bulletin: Methylnaltrexone (254)
The 2009 RGH E-Bulletins are archived here.
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