Methylnaltrexone Information Sheet
Download a printable version of the Methylnaltrexone Information Sheet.
12 mg/0.6 mL solution for injection
Practice points
- Methylnaltrexone is a subsidised option for opioid-induced constipation in palliative care when response to laxative therapy has not been sufficient.
- It is not effective for constipation caused by other medications or by other mechanisms.
- It treats opioid-induced constipation without affecting opioid analgesia.
- The subcutaneous injection can be prescribed, fully subsidised with Special Authority approval, to patients who are receiving palliative care.
- Half of patients will have a bowel movement within four hours of administration, typically within the first 30 to 60 minutes.
- Contradicted in known or suspected mechanical gastrointestinal obstruction or acute surgical abdomen.
Mechanism of action
- Methylnaltrexone is a competitive antagonist of opioid receptor.
- It functions as a peripherally acting mu-opioid antagonist in tissues such as the gastrointestinal tract.
- It does not cross the blood brain barrier so does not impact opioid-mediated analgesic effects on the central nervous system.
- Methylnaltrexone is minimally metabolised in humans and is eliminated mainly as the unchanged drug in urine.
- Half-life is approx. 8 hours.
Dosing
Patient weight (kg)
|
Injection volume
|
Dose
|
38 to less than 62
|
0.4 mL
|
8 mg
|
62 to 114
|
0.6 mL
|
12 mg
|
Less than 38 or more than 114
|
0.0075 mL/kg (rounded to nearest 0.1 mL)
|
0.15 mg/kg (rounded to nearest 2 mg)
|
- In patients with severe renal impairment (creatinine clearance less than 30 mL/min) reduce the dose by one-half.
- No dose adjustment is required in patients with mild or moderate renal impairment.
- There is no data available from patients on dialysis so is not recommended.
Prescribing
- Methylnaltrexone is for subcutaneous infection only.
- Prescribed as a stat dose or as a single dose on alternate days.
- Doses may also be given with longer intervals, as needed.
- It can also be given as a single stat dose and repeated at a later date in needed.
- If there has been no bowel movement within 24 hours of the last dose, an additional dose may be given.
- Ongoing use in the community can be inconvenient so maintaining control using other treatments is recommended.
- Patients can continue taking other treatments for constipation, such as oral laxatives.
Administration
- Inject subcutaneously in the upper arm, abdomen or thigh.
- Move to a different site each time an injection is given.
- Avoid repeated injections at the exact same spot previously used.
- Do not inject into areas where the skin is tender, bruised, red, or hard.
- Avoid areas with scars or stretch marks.
- Each vial is for single use in one patient only and any residue must be discarded.
Contraindications
- Methyl naltrexone is contradicted in known or suspected mechanical gastrointestinal obstruction or acute surgical abdomen.
- It is not recommended for use in post-operative ileus, including patients who have undergone gastrointestinal resection.
Adverse reactions
- Abdominal cramping or abdominal pain are common following treatment.
- Patients who develop severe or persistent abdominal pain that is not relieved by passing a bowel motion should discontinue treatment.
- Main side effects are abdominal pain, flatulence, nausea, dizziness, and diarrhoea.
Special Authority requirements
The patient is receiving palliative care AND:
- Oral or rectal treatments for opioid induced constipation are ineffective,
OR
- Oral or rectal treatments for opioid induced constipation are unable to be tolerated.
https://schedule.pharmac.govt.nz/latest/SA1691.pdf
Advice and support
Within working hours contact the Christchurch Hospital Palliative Care Service on (03) 364 1473, or the Nurse Maude Hospice on (03) 375 4274.
After hours contact the Christchurch Hospital operator on (03) 364 0640 and ask for the palliative care clinician on call.
Topic Code: 951856
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