Methylnaltrexone
Methylnaltrexone is a peripherally acting opioid blocker used to treat opioid-induced constipation in adults with chronic non-cancer pain or advanced illness, depending on the product.
🧬 Where this information comes from Click to expand
The clinical label records used on this page are FDA-filed Structured Product Labeling (SPL) retrieved through openFDA. We identified 1 clinical label set within the page’s selected clinical scope after exact ingredient or ingredient-combination matching and applicable route/form matching. Forms, routes, brands, labelers, strengths, and NDC product-record counts are built separately from matching FDA National Drug Code Directory records. The linked DailyMed document is a representative official source for verification, not the page’s only clinical source. View the linked label on DailyMed →
Methylnaltrexone (Relistor), on the market since about 2008, treats constipation caused by opioid pain medicine by blocking opioids mainly in the gut, and because little reaches the brain, your pain relief stays intact; it is usually tried after ordinary laxatives fall short. Most people tolerate it well, and the usual effects are belly cramping, nausea and a prompt bowel movement. Get help right away for severe, lasting or worsening belly pain, a possible sign of a tear in the bowel wall, and stop it and call for severe diarrhea. Tablets and injection are not approved for exactly the same uses, so follow the plan your prescriber chose.
Clinical pearls
- Take the tablets in the morning with water on an empty stomach, at least 30 minutes before your first meal.
- Stay close to a toilet after each dose, since a bowel movement can come on quickly.
- For chronic non-cancer pain, stop regular laxatives before starting, and use them as needed if results are weak after 3 days.
- Stop it when you stop your opioid, and avoid other opioid blockers, which can trigger withdrawal like sweating, chills and yawning.
Patient information guide A plain-language walkthrough of Methylnaltrexone, written from its FDA label.
What Methylnaltrexone is used for
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Methylnaltrexone treats opioid-induced constipation (OIC) in adults.
- Relistor tablets and Relistor injection: OIC in adults with chronic non-cancer pain, including pain related to prior cancer or its treatment, when opioid doses do not need frequent (e.g., weekly) increases.
- Relistor injection only: OIC in adults with advanced illness or pain from active cancer who need opioid dose increases for palliative care.
How Methylnaltrexone works
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Methylnaltrexone blocks opioids from attaching to mu-opioid receptors. Because it does not cross easily into the brain, it acts mainly in places like the gut.
That helps reverse the constipating effect of opioids while leaving their pain-relieving action in the central nervous system intact.
Methylnaltrexone dosage
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Relistor comes as a tablet and as an injection. Doses differ by product and reason for use, so follow your prescriber and the label.
- Tablets: taken by mouth once daily in the morning, with water, on an empty stomach, at least 30 minutes before the first meal.
- Injection: given under the skin of the upper arm, abdomen or thigh, with sites rotated. It is once daily for chronic non-cancer pain, or every other day as needed for advanced illness.
- Stay near a toilet after a dose.
- Stop it if your opioid pain medicine is stopped.
Dosing details from the FDA-approved label
From the Methylnaltrexone prescribing information (“Dosage and Administration”). Doses are set by your prescriber for your product, condition, kidney function and age — never change a dose on your own.
- OIC in adult patients with chronic non-cancer pain (RELISTOR tablets)
- 450 mg taken orally once daily in the morning
- Take with water on an empty stomach at least 30 minutes before the first meal of the day
- OIC in adult patients with chronic non-cancer pain (RELISTOR injection)
- 12 mg administered subcutaneously once daily
- OIC in adult patients with advanced illness (RELISTOR injection), weight less than 38 kg
- 0.15 mg/kg subcutaneously, one dose every other day, as needed
- Maximum: Do not administer more frequently than one dose per 24-hour period
- Injection volume: multiply patient weight in kg by 0.0075, round up to the nearest 0.1 mL
- OIC in adult patients with advanced illness (RELISTOR injection), weight 38 kg to less than 62 kg
- 8 mg (0.4 mL) subcutaneously, one dose every other day, as needed
- Maximum: Do not administer more frequently than one dose per 24-hour period
- OIC in adult patients with advanced illness (RELISTOR injection), weight 62 kg to 114 kg
- 12 mg (0.6 mL) subcutaneously, one dose every other day, as needed
- Maximum: Do not administer more frequently than one dose per 24-hour period
- OIC in adult patients with advanced illness (RELISTOR injection), weight more than 114 kg
- 0.15 mg/kg subcutaneously, one dose every other day, as needed
- Maximum: Do not administer more frequently than one dose per 24-hour period
- Injection volume: multiply patient weight in kg by 0.0075, round up to the nearest 0.1 mL
- Chronic non-cancer pain OIC with moderate or severe renal impairment (creatinine clearance less than 60 mL/minute)
- Tablets: 150 mg once daily in the morning
- Injection: 6 mg subcutaneously once daily
- Kidney adjustment
- Advanced illness OIC with moderate or severe renal impairment (creatinine clearance less than 60 mL/minute)
- One dose every other day, as needed: less than 38 kg 0.075 mg/kg
- 38 kg to less than 62 kg 4 mg (0.2 mL)
- 62 kg to 114 kg 6 mg (0.3 mL)
- More than 114 kg 0.075 mg/kg
- For 0.075 mg/kg doses, volume is weight in kg x 0.00375, rounded up to the nearest 0.1 mL
- Chronic non-cancer pain OIC with moderate or severe hepatic impairment (Child-Pugh Class B or C), tablets
- 150 mg once daily in the morning
- Hepatic adjustment
- Chronic non-cancer pain OIC with severe hepatic impairment, injection
- If considering dose adjustment: less than 38 kg 0.075 mg/kg
- 38 kg to less than 62 kg 4 mg (0.2 mL)
- 62 kg to 114 kg 6 mg (0.3 mL)
- More than 114 kg 0.075 mg/kg
- For 0.075 mg/kg doses, volume is weight in kg x 0.00375, rounded up to the nearest 0.1 mL
Read the label’s full dosing text
2 DOSAGE AND ADMINISTRATION 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 24-hour period. Dosage Adjustment • See full prescribing information dosage adjustment in renal or hepatic impairment by indication. ( 2.4 , 2.5 ) Preparation and Administration of RELISTOR Injection ( 2.6 ) • For subcutaneous use only. • Inject in upper arm, abdomen or thigh. Rotate injection sites. 2.1 Important Administration Information • Be within close proximity to toilet facilities once RELISTOR is administered. • Discontinue RELISTOR if treatment with the 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 [see Clinical Studies ( 14.1 )]. o Discontinue all maintenance laxative therapy prior to initiation of RELISTOR. Laxative(s) can be used as needed if there is a suboptimal response to RELISTOR after three days.
Excerpted — the section continues on the label. Read the full Dosage and Administration section on DailyMed →
Methylnaltrexone side effects
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The most common side effects include:
Common side effects
- Abdominal pain
- Diarrhea
- Nausea
- Headache (tablets)
- Abdominal bloating or distention (tablets)
- Vomiting (tablets)
- Sweating
- Chills
- Hot flush and tremor (injection, chronic non-cancer pain)
- Gas and dizziness (injection, advanced illness)
When to get medical help
- Seek medical help right away for severe, persistent or worsening belly pain. This could signal a tear in the gut wall (gastrointestinal perforation).
- Stop the medicine and call your healthcare provider if you have severe or persistent diarrhea.
- Call your provider if you notice opioid withdrawal symptoms such as sweating, chills, diarrhea, belly pain, anxiety or yawning.
- Tell your provider if your pain relief seems to be getting weaker.
Some of these may reflect opioid withdrawal. Act on severe or worsening belly pain, severe or persistent diarrhea, or withdrawal symptoms.
Methylnaltrexone warnings and precautions
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- Gastrointestinal perforation: a tear in the gut wall has been reported, mostly in people with weakened bowel walls. Stop and get help for severe, persistent or worsening belly pain.
- Severe or persistent diarrhea: stop and talk with your provider.
- Opioid withdrawal: symptoms include sweating, chills, diarrhea, belly pain, anxiety and yawning. Risk may be higher with a disrupted blood-brain barrier.
Methylnaltrexone interactions
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Few interactions are described, but one matters.
- Other opioid antagonists: avoid combining, because effects add up and raise withdrawal risk.
- Maintenance laxatives: stop before starting if you have chronic non-cancer pain. They may be used as needed if the response is poor after 3 days.
- Dextromethorphan: methylnaltrexone did not significantly affect its metabolism in healthy people.
Interactions listed in the FDA-approved label
From the Methylnaltrexone prescribing information (“Drug Interactions”): what the manufacturer studied or reported to the FDA.
- Other opioid antagonists: Potential for additive effects of opioid receptor antagonism and increased risk of opioid withdrawal. Avoid concomitant use.
- Drugs metabolized by cytochrome P450 isozymes (dextromethorphan, a CYP2D6 substrate): In healthy subjects, a subcutaneous dose of 0.3 mg/kg of RELISTOR did not significantly affect the metabolism of dextromethorphan.
Read the label’s full interactions text
7 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.
Not a complete list. The label covers the interactions the manufacturer studied or reported, and our own interaction database does not cover every medicine either. Bring your full medication list, including vitamins and herbal products, to your pharmacist.
Before taking Methylnaltrexone
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- Do not use with a known or suspected bowel blockage or high risk of recurrent blockage.
- Be cautious with ulcers, diverticular disease, Crohn's disease or gut tumors.
- Pregnancy: may cause opioid withdrawal in a fetus.
- Breastfeeding is not recommended.
- Not established in children.
- Older adults: no age-based dose change; monitor for side effects.
- Moderate or severe kidney impairment calls for a lower dose.
- Tablets: moderate or severe liver impairment calls for a lower dose. Injection: severe liver impairment was not studied.
Methylnaltrexone overdose
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In trials, one accidental overdose caused no problems. A high intravenous dose in healthy people caused orthostatic hypotension (a drop in blood pressure on standing). If someone on opioids takes too much, they should be watched for withdrawal signs such as chills, runny nose, sweating or loss of pain relief, and treated based on symptoms.
What Methylnaltrexone does in the body
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Methylnaltrexone blocks mu-opioid receptors outside the brain. In a heart-rhythm study at doses well above those recommended, it did not prolong the QTc interval to any clinically relevant extent.
How your body processes Methylnaltrexone
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Relistor tablets peak in the blood at about 1.5 hours, and a high-fat meal lowers absorption. Relistor injection peaks at about 0.5 hours. Methylnaltrexone is mostly cleared unchanged in urine and feces, with limited metabolism. The tablet half-life is about 15 hours.
How to store Methylnaltrexone
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Tablets Store at up to 25°C (77°F); excursions permitted to 15° to 30°C (59° to 86°F) . Store at 20° to 25°C (68° to 77°F); excursions permitted to 15° to 30°C (59° to 86°F) . Do not freeze.
Written from the FDA-approved label with AI and reviewed by our pharmacy team. How we use AI →
Supplements & herbs that interact with Methylnaltrexone
3 supplements and herbal products have documented interactions with Methylnaltrexone in the licensed clinical database we use. Most are manageable — but your pharmacist should know about everything you take, including vitamins and herbals.
Educational information from a licensed clinical database (Natural Medicines) — see our data sources & update cadence. Not medical advice: an interaction being documented does not mean it will happen to you; do not start or stop medications or supplements without professional guidance.
Methylnaltrexone: pharmacist answers to common questions
Quick, plain-English answers to the questions patients ask most about Methylnaltrexone — the kind of thing our pharmacy team would talk through with you at the counter.
What is methylnaltrexone for?
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Will it take away my pain relief?
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How do I take it?
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What side effects should I expect?
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When should I get help right away?
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Can I use it if I'm pregnant or breastfeeding?
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Is Methylnaltrexone available over the counter?
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When was Methylnaltrexone first available?
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Answers drafted from the FDA-approved label with AI and reviewed by our pharmacy team. How we use AI →
More about Methylnaltrexone on HelloPharmacist
Methylnaltrexone forms and strengths (how it comes)
Methylnaltrexone is available in 2 forms. Different forms and brands can have different approved uses, doses, schedules, and directions. Follow the instructions for your exact product, and do not switch forms without guidance from your prescriber or pharmacist.
Injection
How this form is used
- Relistor injection treats opioid-induced constipation in adults with chronic non-cancer pain.
- Relistor injection also treats opioid-induced constipation in adults with advanced illness or pain from active cancer who need opioid dose increases for palliative care.
- Relistor injection is for subcutaneous use only, in the upper arm, abdomen or thigh.
- Rotate injection sites with Relistor injection.
- Relistor injection should look colorless to pale yellow; it should not be used if particles or discoloration are present.
- For advanced illness, Relistor injection is given every other day as needed, no more than once in 24 hours.
Tablet
How this form is used
- Relistor tablets treat opioid-induced constipation in adults with chronic non-cancer pain, including pain related to prior cancer or its treatment.
- Take Relistor tablets once daily in the morning.
- Take Relistor tablets with water on an empty stomach at least 30 minutes before the first meal of the day.
- Stay close to a toilet after taking Relistor tablets.
Methylnaltrexone on the U.S. market: products, makers and brands
How Methylnaltrexone appears in the FDA’s National Drug Code Directory — including its dosage forms, strengths, brand names, manufacturers, and FDA-listed product records. These are directory records, not sales or prescription volume.
1 company lists 3 Methylnaltrexone product records with the FDA, mostly as injection, tablet; the earliest marketing or approval year on record is 2008. Brand names on the directory include Relistor; the rest are generics.
FDA-listed product records by dose form (tablet variants merged) — FDA National Drug Code Directory. Record counts are not sales or prescription volume.
Also listed with the FDA, not a patient dose form: Powder (6) — bulk drug substance sold to compounders/manufacturers and multi-item kits. These aren’t counted in the dose-form total above.
How widely Methylnaltrexone is used (Medicaid data)
A general measure of how commonly Methylnaltrexone is prescribed, based on Medicaid — one of the largest public drug programs. The figure below is prescriptions filled in Q1–Q4 2025, summed across every form and brand of the drug.
Think of this as a proxy for how widely Methylnaltrexone is used, drawn from freely available public data. Medicaid is just one program — these numbers don’t include Medicare, other government coverage, or commercial and cash-pay prescriptions, so real-world use is higher than what’s shown here.
How commonly is Methylnaltrexone prescribed? Of the 1,601 medications we measure, Methylnaltrexone ranks #805 by Medicaid prescriptions — more than 50% of them.
Utilization by Methylnaltrexone product
Pick a product to see its own Medicaid numbers. Each chip is one clinical product — a specific strength & dosage form as defined by RxNorm — with brand-name and generic versions combined.
methylnaltrexone bromide 150 MG Oral Tablet Most dispensed
Summed across the 1 of 4 listed NDC products with Medicaid activity — every brand and generic of this exact strength & form. RxNorm 1801514
0.6 ML methylnaltrexone bromide 20 MG/ML Prefilled Syringe
Summed across the 2 of 2 listed NDC products with Medicaid activity — every brand and generic of this exact strength & form. RxNorm 1244060
0.6 ML methylnaltrexone bromide 20 MG/ML Injection
Summed across the 1 of 1 listed NDC products with Medicaid activity — every brand and generic of this exact strength & form. RxNorm 979113
0.4 ML methylnaltrexone bromide 20 MG/ML Prefilled Syringe
Summed across the 1 of 1 listed NDC products with Medicaid activity — every brand and generic of this exact strength & form. RxNorm 1244063
Shares are of the Medicaid prescriptions shown here. Product grouping follows RxNorm (the National Library of Medicine’s drug terminology), so “one product” means one strength & dosage form regardless of manufacturer. Dollar figures are gross Medicaid reimbursement before mandatory rebates — rebates (often large, especially for brand-name drugs) are confidential, so actual net cost to Medicaid is lower than shown.
Source: Medicaid State Drug Utilization Data (CMS), aggregated by generic ingredient — summed across every NDC (all brands, strengths, salt forms and dosage forms) of Methylnaltrexone, and across all states and both fee-for-service and managed-care claims. A general popularity signal; it excludes Medicare, commercial insurance and cash prescriptions, so it is not total U.S. use. Based on the 5 of 8 listed Methylnaltrexone NDCs with Medicaid activity.
Compare Methylnaltrexone products
A representative set of FDA-listed product records for Methylnaltrexone — across manufacturers, strengths, and dosage forms, grouped by form with each product’s manufacturer and how the FDA approved it. The same medicine may be made and packaged by different companies, so a single drug can have many directory entries. (It’s also why a refill can look different — a new shape, color, or box — even though it contains the same medication.)
| Strength | Route | Manufacturer / Labeler | Category ⓘ | Details |
|---|---|---|---|---|
| 💊Injection | ||||
| 8 mg/0.4 mL | Subcutaneous | Salix Pharmaceuticals, Inc. | NDA | View NDC → |
| 12 mg/0.6 mL | Subcutaneous | Salix Pharmaceuticals, Inc. | NDA | View NDC → |
| 💊Tablet | ||||
| 150 mg | Oral | Salix Pharmaceuticals, Inc. | NDA | View NDC → |
Showing 3 of 3 products — FDA National Drug Code Directory. See every product, package size and price: browse all 3 Methylnaltrexone NDCs →
Methylnaltrexone side effects reported to the FDA (FAERS)
After a medicine reaches the market, patients, doctors and drugmakers can send the FDA reports of problems they think may be linked to it. Here’s what’s been reported for Methylnaltrexone — a signal of what to watch for, not proof the drug caused it.
Source: openFDA, from the FDA Adverse Event Reporting System (FAERS). These are voluntary reports — they don’t prove the drug caused the effect, and counts reflect how often something was reported, not how often it actually happens. Reports also often name the very problem the medicine is taken for — a common reason to file one is that the drug didn’t help — so for a pain reliever you may see “pain” high on the list; that points to why the report was filed, not to the drug causing the symptom. This counts every report that lists Methylnaltrexone in any role, so the total runs higher than the FDA’s official dashboard, which counts only cases where the drug is the suspect. For the authoritative figures, see the FDA FAERS Public Dashboard. Always talk to your pharmacist or doctor.
Methylnaltrexone FDA approval history
How Methylnaltrexone went from FDA review to the pharmacy shelf — the key milestones in its regulatory journey.
Source: Drugs@FDA.
Methylnaltrexone recall history
A recall is when a specific batch of a medicine is pulled from the market — usually a manufacturing issue, not a problem with the drug itself. Class I is most serious, Class III least.
The FDA’s enforcement reports list no Methylnaltrexone recalls since July 2021.
✅No Methylnaltrexone recalls on record since July 2021Source: FDA enforcement (recall) reports, via openFDA; our copy was last updated Sep 24, 2026.
RxNorm Concept Web — From Ingredient to Every Form and Strength
This page starts with one ingredient concept (IN). The branches below show its dose-form concepts (SCDF), then the available clinical drug and strength concepts (SCD). Combination products remain separate concepts with their own pages.
RxCUI 29899 3 dose forms · 4 strengths
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Dose form (SCDF) Injection RxCUI 1659739In current FDA listingsClinical drug / strength (SCD) 0.6 ML methylnaltrexone bromide 20 MG/MLRxCUI 979113 -
Dose form (SCDF) Oral Tablet RxCUI 1801513In current FDA listingsClinical drug / strength (SCD) bromide 150 MGRxCUI 1801514 -
Dose form (SCDF) Prefilled Syringe RxCUI 1244059No current FDA listingClinical drug / strength (SCD) 0.4 ML methylnaltrexone bromide 20 MG/MLRxCUI 12440630.6 ML methylnaltrexone bromide 20 MG/MLRxCUI 1244060
Look up RxCUI 29899 in the RxCUI Atlas →
RxNorm is the U.S. National Library of Medicine’s normalized drug vocabulary. Its concept hierarchy can include forms that are not currently marketed; the status on each branch compares that concept with current FDA listings. RxCUIs identify vocabulary concepts, not individual packages or manufacturers.
Methylnaltrexone supply and shortage status
Whether Methylnaltrexone is in short supply in the U.S. right now, plus any shortage history the FDA has on record. A shortage usually reflects a manufacturing or demand issue — not a safety problem with the drug.
Source: openFDA, from the FDA Drug Shortages database.
💵 What Methylnaltrexone costs pharmacies (NADAC benchmark)
The national average invoice price pharmacies pay for the drug itself, from CMS's NADAC survey (data as of Feb 21, 2024). This is a supply-cost benchmark — not a retail price and not what you'd pay at the counter.
| Form | Strength | Pharmacy acquisition benchmark | |
|---|---|---|---|
| Injection, Solution · Subcutaneous | 12 mg/0.6 mL | $254.66–$256.47 per mL | NDC details & price history |
| Tablet · Oral | 150 mg | $25.34 per tablet | NDC details & price history |
Source: CMS National Average Drug Acquisition Cost (NADAC), a survey of retail pharmacy invoice prices. Benchmarks shown are the range across package sizes of a representative product for each form and strength — other manufacturers' products may differ.
Methylnaltrexone brand names
Methylnaltrexone is sold under one brand name in the FDA directory — Relistor. A brand and its generic contain the same active ingredient; the brand name belongs to one company's product.
Who makes Methylnaltrexone: manufacturers and labelers
One company lists Methylnaltrexone products with the FDA. By number of product records, the largest are Salix Pharmaceuticals, Inc.. Labelers include manufacturers and repackagers; the product in your bottle depends on which one your pharmacy stocks.
Methylnaltrexone drug class (RxNorm)
Methylnaltrexone belongs to the Opioid Antagonist class.
Classified by RxNorm RxClass — the U.S. National Library of Medicine's standardized drug-classification service (Established Pharmacologic Class from the FDA, the ATC drug family from the WHO, and mechanism of action). Reference only, not medical advice.
Sources for this Methylnaltrexone page
Core label, product, RxNorm, safety, and utilization data on this page come from the sources identified below. AI-written, editorial, and licensed sections are labeled separately.
How this page is built
HelloPharmacist combines public FDA and NLM records and does not author the underlying clinical facts. The plain-language sections are written with AI from the FDA-approved label text and reviewed by our pharmacy team before publication. Salix Pharmaceuticals, Inc. is the representative DailyMed label linked for verification and dates. This is general education, not medical advice: always talk to your doctor or pharmacist about your own medicines. How we build and review drug pages →