Methylnaltrexone
Methylnaltrexone (Relistor) is a targeted opioid antagonist used to treat opioid-induced constipation in adults with chronic non-cancer pain or advanced illness.
🧬 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: Patient Information Guide
A plain-language walkthrough of Methylnaltrexone — what it treats, how it works, how it’s taken, side effects, warnings and interactions. Tap any section to expand it.
🎯What it's used for▾
Methylnaltrexone (Relistor) is specifically approved to treat opioid-induced constipation (OIC) — constipation that develops as a side effect of opioid pain medications.
- Relistor tablets and Relistor injection are both approved for adults with chronic non-cancer pain, including those whose pain is related to a prior cancer or its treatment, as long as they do not need frequent (e.g., weekly) increases in their opioid dose.
- Relistor injection only is approved for adults with advanced illness or active cancer pain who require escalating opioid doses as part of palliative (comfort-focused) care.
⚙️How it works▾
Opioid pain medications work in the brain to relieve pain, but they also bind to receptors in the gut — slowing movement through the digestive tract and causing constipation. Methylnaltrexone is designed to block those same receptors specifically in the gut. Because of its chemical structure (it's a quaternary amine), it cannot easily cross the blood-brain barrier, so it relieves constipation without canceling out the pain-relieving effect of the opioid in the brain.
💊How it's taken▾
Methylnaltrexone is available as a tablet taken by mouth and as an injection given under the skin. Relistor tablets are taken once daily in the morning with water, on an empty stomach — at least 30 minutes before eating, because a high-fat meal can significantly reduce how much drug is absorbed. Relistor injection is given once daily for chronic non-cancer pain, or every other day as needed (no more than once every 24 hours) for advanced illness. Doses of Relistor injection for advanced illness are weight-based; your healthcare provider will determine the right amount. Your prescriber may also adjust the dose if you have moderate or severe kidney disease or, for the tablet form, moderate or severe liver disease. Always follow your prescriber's specific instructions.
🤒Side effects — in detail▾
The most commonly reported side effects of Relistor include:
- Abdominal pain or cramping
- Diarrhea
- Nausea
- Sweating (hyperhidrosis)
- Chills
- Headache (more common with tablets)
- Abdominal bloating or distention (more common with tablets)
- Vomiting
- Hot flashes and tremor (more common with injection)
- Flatulence and dizziness (reported in advanced illness patients receiving injection)
Some of these — especially sweating, chills, diarrhea, stomach pain, anxiety, and runny nose — may actually be signs of opioid withdrawal. If they are severe or don't go away, stop the medication and call your doctor right away. A serious gut tear (gastrointestinal perforation) is rare but has been reported — severe, persistent abdominal pain is the key warning sign.
⚠️Warnings & precautions▾
- Gut perforation (gastrointestinal perforation): A rare but serious tear in the wall of the intestine has occurred, mainly in patients with advanced illness who also have conditions that weaken the gut wall (such as peptic ulcer disease, diverticular disease, or Crohn's disease). Stop Relistor and get medical help immediately if you develop severe or worsening abdominal pain.
- Opioid withdrawal: Relistor can trigger withdrawal symptoms — sweating, chills, diarrhea, stomach pain, anxiety, yawning — especially in people with a disrupted blood-brain barrier. If you notice these symptoms or feel your pain relief has weakened, contact your doctor promptly.
- Severe or persistent diarrhea: If diarrhea is severe or does not improve, stop taking Relistor and contact your healthcare provider.
- Do not use if you have a bowel blockage: Relistor is contraindicated in anyone with a known or suspected gastrointestinal obstruction, due to the risk of perforation.
🔀What it interacts with▾
Relistor has a small number of known drug interactions to be aware of:
- Other opioid antagonists (e.g., naltrexone, naloxone): Avoid combining these with Relistor. Using them together can amplify opioid-blocking effects and sharply increase the risk of opioid withdrawal.
- Stopping your opioid medication: If you stop taking your opioid pain medication, you should also stop Relistor — it is only needed while you are on opioids.
This is not a complete list of interactions. Always talk with your doctor or pharmacist before starting, stopping, or changing any medication.
📋Before taking it▾
- Bowel blockage: Do not take Relistor if you have or may have a gastrointestinal obstruction — it can cause a dangerous perforation.
- Gut conditions: Tell your doctor if you have peptic ulcer disease, Crohn's disease, diverticular disease, or cancer involving the gut — these conditions raise the risk of a serious gut tear.
- Kidney disease: Moderate or severe kidney disease increases drug levels in your body; a dose reduction is recommended for both Relistor tablets and injection.
- Liver disease: Moderate or severe liver disease increases levels of Relistor tablets; a dose reduction is recommended. For Relistor injection, no adjustment is needed for mild or moderate liver disease, but severe liver disease requires careful monitoring.
- Pregnancy: Relistor may cause opioid withdrawal in the fetus, whose blood-brain barrier is not yet fully developed. Discuss the risks with your doctor.
- Breastfeeding: Methylnaltrexone passes into animal milk, and breastfeeding is not recommended during treatment due to potential harm to the infant.
- Children: Safety and effectiveness have not been established in pediatric patients.
- Older adults: Relistor appears to work similarly in older adults, but a higher rate of diarrhea has been seen — your doctor should monitor you.
- Blood-brain barrier disruption: People with conditions that affect the blood-brain barrier may be at higher risk of opioid withdrawal or reduced pain control — discuss this with your prescriber.
🚑If you take too much▾
Overdose experience with Relistor is very limited, but a study in healthy subjects found that a high intravenous dose caused dizziness when standing up (orthostatic hypotension — a drop in blood pressure upon standing). If someone on opioid therapy takes too much Relistor, they should be monitored closely for signs of opioid withdrawal — such as chills, runny nose, sweating, changes in blood pressure or heart rate, or reduced pain relief. Treatment is based on the severity of withdrawal symptoms and the person's ongoing need for pain control.
📡Pharmacodynamics — effects in the body▾
Methylnaltrexone blocks mu-opioid receptors in the gut, reversing the slowing effect that opioids have on the digestive tract — without blocking pain relief in the brain. Studies show that even at doses much higher than the recommended amount, methylnaltrexone does not meaningfully affect the heart's electrical rhythm (specifically, it does not prolong the QTc interval to a clinically relevant degree).
🔬Pharmacokinetics — how your body processes it▾
When taken as Relistor tablets, methylnaltrexone reaches peak blood levels in about 1.5 hours; taking it with a high-fat meal significantly reduces and delays absorption, which is why it should be taken on an empty stomach. Relistor injection reaches peak levels much faster — within about 30 minutes of the subcutaneous injection. The drug is broken down into several metabolites, some of which are also active, and is eliminated primarily unchanged through the urine and feces. The half-life (how long it takes the body to clear half the drug) after oral dosing is approximately 15 hours. Patients with moderate or severe kidney disease clear the drug more slowly, which is why a lower dose is recommended for them.
📦How to store it▾
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 Methylnaltrexone’s FDA-approved label information, summarized in plain language with AI, and reviewed by our pharmacy team before publication. Every point is grounded in that labeling — nothing is invented. 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.
🩺 Pharmacist Counseling Corner
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.
Will Relistor make my opioid pain medication stop working? ▾
How quickly will Relistor work? ▾
What side effects should I actually watch out for? ▾
I take Relistor tablets — does it matter if I eat first? ▾
Can I take Relistor if I'm pregnant or breastfeeding? ▾
Do I need to stop taking Relistor if my doctor changes or stops my opioid? ▾
Is Methylnaltrexone available over the counter? ▾
When was Methylnaltrexone first available? ▾
Who makes Methylnaltrexone? ▾
💬 Answers drafted from Methylnaltrexone’s FDA-approved label information, summarized in plain language with AI, and reviewed by our pharmacy team before publication. Grounded in that labeling — nothing is made up. How we use AI →
🧰 Keep exploring
💊 How Methylnaltrexone 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 is used to treat opioid-induced constipation in adults with chronic non-cancer pain, including those with pain from prior cancer or its treatment who do not need frequent opioid dose increases
- Relistor injection is also used to treat opioid-induced constipation in adults with advanced illness or active cancer pain who require increasing opioid doses for palliative care
- Relistor injection is given as a subcutaneous (under-the-skin) injection in the upper arm, abdomen, or thigh — rotate the injection site each time
- For chronic non-cancer pain, Relistor injection is given once daily; for advanced illness, it is given every other day as needed, no more than once in any 24-hour period
- Relistor injection is for subcutaneous use only — do not inject it intravenously
- Stay near a bathroom after receiving Relistor injection
Tablet
How this form is used
- Relistor tablets are used to treat opioid-induced constipation in adults with chronic non-cancer pain, including those with pain from prior cancer or its treatment who do not need frequent opioid dose increases
- Relistor tablets are taken by mouth once daily in the morning
- Relistor tablets should be taken with water on an empty stomach, at least 30 minutes before the first meal of the day
- Avoid high-fat meals before taking Relistor tablets, as food significantly reduces absorption
- Stay near a bathroom after taking Relistor tablets
📊 Methylnaltrexone across the U.S. market
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.
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.
Share of FDA-listed product records by manufacturer — FDA National Drug Code Directory. Record counts are not sales or prescription volume.
📈 How widely Methylnaltrexone is used
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 1,596 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. × 3 listings | NDA | View NDC → |
| 12 mg/0.6 mL | Subcutaneous | Salix Pharmaceuticals, Inc. × 3 listings | NDA | View NDC → |
| 💊Tablet | ||||
| 150 mg | — | Patheon Inc. | DRUG FOR FURTHER PROCESSING | View NDC → |
| 150 mg | Oral | Salix Pharmaceuticals, Inc. × 3 listings | NDA | View NDC → |
| 💊Bulk drug substance | ||||
| 1 kg/kg | — | Noramco, LLC | BULK INGREDIENT | View NDC → |
| 35 kg/35 kg | — | Nuray Chemicals Private Limited | BULK INGREDIENT | View NDC → |
| 1 kg/kg | — | Pcas | BULK INGREDIENT | View NDC → |
| 50 kg/50 kg | — | SpecGx LLC | BULK INGREDIENT | View NDC → |
| 35 kg/35 kg | — | Sthree Chemicals Private Limited | BULK INGREDIENT | View NDC → |
| 25 kg/25 kg | — | Sun Pharmaceutical Industries, Inc. | BULK INGREDIENT | View NDC → |
Showing 16 of 16 products — FDA National Drug Code Directory. See every product, package size and price: browse all 16 Methylnaltrexone NDCs →
📈 What people report — real-world data (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.
🏛️ The road to your pharmacy
How Methylnaltrexone went from FDA review to the pharmacy shelf — the key milestones in its regulatory journey.
Source: Drugs@FDA.
🚨 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.
Source: openFDA drug enforcement (recall) reports.
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
-
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.
📦 Supply & 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.
🏷️ Sold under these brand names
🏭 Manufacturers & labelers
RxNorm drug class
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.
🔬 Where this comes from
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 we combine label and product data
HelloPharmacist combines public FDA and NLM records; we don’t author the underlying clinical facts. We identified 1 FDA-filed SPL label set within the page’s selected clinical scope after exact ingredient or ingredient-combination matching and applicable route/form matching. The representative label is the starting point for the displayed reference monograph; the 12 largest stored in-scope SPL records, which may include that representative, are compared section by section, and the longest available version of each section is retained. Forms, routes, brands, labelers, product-listed strengths, and product-record counts are aggregated separately from ingredient-matched FDA NDC Directory records; package records listed above are associated with the linked SPL; and the normalized concept hierarchy comes from NLM RxNorm.
For canonical ingredient pages, bounded product-attributed indication and administration excerpts may be added to the AI evidence, and the generator is instructed to preserve available product, brand, route, and form qualifiers instead of treating one label as universal. Where a section is identified as AI-written, its plain-language text is generated from bounded label evidence. New draft drug pages remain in the admin review queue before first publication. Salix Pharmaceuticals, Inc. is the representative DailyMed label linked for verification and dates; it is not the sole source used for the page.
This is general education, not medical advice — always consult your doctor or pharmacist about your medications and any medical condition. For the exact wording of the linked representative label, view that label on DailyMed; some displayed composite sections may come from other in-scope SPL labels. Learn how we use AI and our data sources & update cadence.