Methylnaltrexone bromide

How it works

As a quaternary amine it does not readily cross the blood-brain barrier, so it antagonises mu-opioid receptors in the gut wall to reverse opioid effects on bowel motility without compromising central analgesia.

Prescribing in practice

• Contraindicated where mechanical gastrointestinal obstruction is known or suspected, owing to the risk of perforation.
• It is given by subcutaneous injection in many indications, with oral and intravenous formulations available depending on the product.
• Discontinue if severe or persistent diarrhoea occurs, and review if opioid therapy is stopped.

Monitoring

Monitor for bowel response, abdominal pain and the development of severe diarrhoea, and remain alert for signs of gastrointestinal perforation.

Counselling the patient

• Tell patients the medicine often produces a bowel movement quite quickly and to stay near a toilet after a dose.
• Advise patients to stop the medicine and seek urgent help if they develop severe, persistent abdominal pain.

Evidence & guidelines

Use of peripherally acting mu-opioid receptor antagonists for opioid-induced constipation is supported by NICE guidance and the relevant SPCs.