Methylnaltrexone: uses, dosing, side effects & brands
Methylnaltrexone is an opioid antagonist sold in the U.S. under one brand, for constipation. Below: what the FDA label says, every product that contains it, what the pills look like, and its recall record.
Key facts
- Drug class
- Opioid Antagonist
- Treats (across its forms)
- Constipation
- Available as
- Injectable · Tablet
- Sold as
- Relistor
- Prescription?
- Prescription only
- Generic available?
- Not in our catalog
How methylnaltrexone is dosed
From the FDA label for Relistor (application NDA208271). Other methylnaltrexone products — different forms, different strengths — are dosed differently. Follow the label for the one you were prescribed.
Administration Instructions ( 2.1 ) • Be within close proximity to toilet facilities once administered. • Discontinue if treatment with opioid pain medication is also discontinued. • In adult patients with chronic non-cancer pain and OIC: o Patients receiving opioids for less than 4 weeks may be less responsive to RELISTOR. o Discontinue all maintenance laxative therapy before starting RELISTOR; may resume laxatives if there is a suboptimal response to RELISTOR after 3 days. o Re-evaluate the continued need for RELISTOR when opioid regimen is changed to avoid adverse reactions. • In patients with chronic non-cancer pain and OIC, take RELISTOR tablets with water on an empty stomach at least 30 minutes before the first meal of the day. Dosing • For OIC in adult patients with chronic non-cancer pain ( 2.2 ): o RELISTOR tablets: The recommended dosage is 450 mg once daily in the morning. o RELISTOR injection: The recommended dosage is 12 mg subcutaneously once daily. • For OIC in adult patients with advanced illness ( 2.3 ): o The pre-filled syringe is only for patients who require a RELISTOR injection dose of 8 mg or 12 mg. Use the vial for patients who require other doses of RELISTOR injection. o RELISTOR injection: See Table 1 in the full prescribing information for the recommended dosage; administer one dose every other day, as needed, but no more frequently than one dose in a…
Methylnaltrexone side effects
Serious and important adverse reactions described elsewhere in the labeling include: • Gastrointestinal perforation [see Warnings and Precautions ( 5.1 )] • Severe or persistent diarrhea [see Warnings and Precautions ( 5.2 )] • Opioid withdrawal [see Warnings and Precautions ( 5.3 )] The most common adverse reactions are: OIC in adult patients with chronic non-cancer pain ( 6.1 ) • RELISTOR tablets (≥ 2%): abdominal pain, diarrhea, headache, abdominal distention, vomiting, hyperhidrosis, anxiety, muscle spasms, rhinorrhea, and chills. • RELISTOR injection (≥ 1%): abdominal pain, nausea, diarrhea, hyperhidrosis, hot flush, tremor, and chills. OIC in adult patients with advanced illness ( 6.1 ) • RELISTOR injection (≥ 5%): abdominal pain, flatulence, nausea, dizziness, and diarrhea. To report SUSPECTED ADVERSE REACTIONS, contact Salix Pharmaceuticals at 1-800-321-4576 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in clinical practice. Opioid-Induced Constipation in Adult Patients with Chronic Non-Cancer Pain The safety of RELISTOR tablets was evaluated in a double-blind, placebo-controlled trial in…
Who shouldn’t take methylnaltrexone
RELISTOR is contraindicated in patients with known or suspected gastrointestinal obstruction and patients at increased risk of recurrent obstruction, due to the potential for gastrointestinal perforation [see Warnings and Precautions ( 5.1 )] . Patients with known or suspected mechanical gastrointestinal obstruction and at increased risk of recurrent obstruction. ( 4 , 5.1 )
Methylnaltrexone drug interactions
Other Opioid Antagonists : Potential for additive effect and increased risk of opioid withdrawal; avoid concomitant use. ( 7.1 ) 7.1 Other Opioid Antagonists Avoid concomitant use of RELISTOR with other opioid antagonists because of the potential for additive effects of opioid receptor antagonism and increased risk of opioid withdrawal. 7.2 Drugs Metabolized by Cytochrome P450 Isozymes In healthy subjects, a subcutaneous dose of 0.3 mg/kg of RELISTOR did not significantly affect the metabolism of dextromethorphan, a CYP2D6 substrate. 7.1 Other Opioid Antagonists Avoid concomitant use of RELISTOR with other opioid antagonists because of the potential for additive effects of opioid receptor antagonism and increased risk of opioid withdrawal. 7.2 Drugs Metabolized by Cytochrome P450 Isozymes In healthy subjects, a subcutaneous dose of 0.3 mg/kg of RELISTOR did not significantly affect the metabolism of dextromethorphan, a CYP2D6 substrate.
Every methylnaltrexone product we track (1)
Same active ingredient — different manufacturer, form, price and FDA recall record. That last one is what our independent score measures.
Only one: Relistor.
What methylnaltrexone pills look like
Imprint codes, colour and shape from the FDA’s labelling data. Match the imprint on your pill — or search any imprint.
| Imprint | Strength | Colour | Shape | Maker |
|---|---|---|---|---|
| REL;PLAIN | 150 mg | white | round | — |
What people report to the FDA about methylnaltrexone
The FDA Adverse Event Reporting System (FAERS) collects reports from patients and clinicians. It holds 43 reports naming methylnaltrexone, and the FDA flagged 91% of those reports as serious. The effects reported most often — leaving out reports about overdose, misuse or the condition being treated, which dominate the raw list for common medicines:
- abdominal pain8 reports
- vomiting6 reports
- blood creatinine increased3 reports
- constipation3 reports
- hypertension3 reports
- ileus3 reports
- malaise3 reports
- nausea3 reports
Read these as a signal, not a rate. A report does not mean methylnaltrexone caused the effect — anyone can file one, and many describe people taking several medicines for several conditions. Crucially there is no denominator: FAERS does not record how many people took the drug, so these counts cannot be turned into “X% of patients” — a bigger number often just means a more widely used or more talked-about drug. Duplicates exist, and publicity drives reporting. For what is actually established, read the FDA label section above.
Source: openFDA drug/event (FAERS), retrieved July 25, 2026.
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Frequently asked questions
What is methylnaltrexone?
Relistor (Methylnaltrexone Bromide) is an opioid antagonist used to treat Constipation.
What kind of drug is methylnaltrexone?
The FDA classifies methylnaltrexone as an opioid antagonist. Opioid antagonists attach to the same brain and body receptors that opioids use, but without switching them on. By occupying those receptors, they block opioids from binding and can rapidly reverse an overdose or curb opioid cravings. If you are checking whether it is safe to combine with something else, the class is what matters — two drugs from the same class usually should not be stacked.
Can you take methylnaltrexone with other medicines?
It depends on the medicine. We check it against the FDA labels rather than guessing: our interaction checker searches each drug's own label for the other and quotes what it says, naming the section it came from. Run methylnaltrexone against whatever else you take — and remember that a label not naming a drug is not the same as that combination being safe.
What forms does methylnaltrexone come in?
Across the brands we track, methylnaltrexone is currently marketed as injectable and tablet, per the FDA's National Drug Code Directory. Each form is dosed differently — follow the label for the exact product you were prescribed.
Is there a generic methylnaltrexone?
We do not currently list a generic-labelled methylnaltrexone product. That does not always mean none exists — it means none appears under a generic name in the FDA data we track. Ask your pharmacist.
Cite this page
- APA
- pharmaranks. (2026, July 24). Methylnaltrexone: uses, dosing, side effects & brands. https://pharmaranks.com/drugs/methylnaltrexone
- MLA
- “Methylnaltrexone: uses, dosing, side effects & brands.” pharmaranks, 24 July 2026, https://pharmaranks.com/drugs/methylnaltrexone.
We summarise public FDA and NIH sources — for a clinical claim, cite the primary source we link to as well.
Sources: FDA openFDA drug label, National Drug Code Directory, and Enforcement (recall) database. This page reproduces public FDA data and is not medical advice. Dosing is set by your prescriber.
Read the full FDA label for methylnaltrexone on DailyMed (NIH) ↗.