What the label lists and what reports show
Methylnaltrexone side effects
Methylnaltrexone's FDA label: “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.” FAERS holds 1,497 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is nausea (117 reports).
- No generic listed
Check it at the source: the methylnaltrexone FDA label on DailyMed (effective May 1, 2024, Salix Pharmaceuticals, Inc.); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of RELISTOR?See “What is the most important information I should know about RELISTOR?” • The most common side effects of RELISTOR tablets in people with long-lasting (chronic) pain that is not caused by cancer include: stomach-area (abdomen) pain, diarrhea, headache, swelling or a feeling of fullness or pressure in your abdomen, sweating, anxiety, muscle spasms, runny nose, and chills. • The most common side effects of RELISTOR injection in people with long-lasting (chronic) pain that is not caused by cancer include: stomach-area (abdomen) pain, nausea, diarrhea, sweating, hot flush, tremor, and chills. • The most common side effects of RELISTOR injection in people receiving treatment for their advanced illness include: stomach-area (abdomen) pain, gas, nausea, dizziness, and diarrhea. Tell your healthcare provider if you have any side effect that bothers you or that does not go away. […]
Source: FDA drug label, Salix Pharmaceuticals, Inc., effective May 1, 2024 (SPL set c488fb7c-0a5b-487c-b452-996809d1cb99), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe 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.
Table 4: Adverse Reactions* in 4-Week Double-Blind, Placebo-Controlled Period of Clinical…
| Adverse Reaction | RELISTOR Tablets n = 200 | Placebo n = 201 |
|---|---|---|
| Abdominal Pain | 14% | 10% |
| Diarrhea | 5% | 2% |
| Headache | 4% | 3% |
| Abdominal Distention | 4% | 2% |
| Vomiting | 3% | 2% |
| Hyperhidrosis | 3% | 1% |
| Anxiety | 2% | 1% |
| Muscle Spasms | 2% | 1% |
| Rhinorrhea | 2% | 1% |
| Chills | 2% | 0% |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedSerious 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. …
Source: FDA drug label, Salix Pharmaceuticals, Inc., effective May 1, 2024 (SPL set c488fb7c-0a5b-487c-b452-996809d1cb99), via openFDA; the full label on DailyMed
What do FAERS reports show?
1,497 reports list methylnaltrexone among the medicines taken, each a report of something that happened, not proof the medicine caused it; 332 are coded as a death and 339 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 20 reactions below, 10 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Methylnaltrexone: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions