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Side Effects


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.

The safety of RELISTOR was evaluated in two, double-blind, placebo-controlled trials in patients with advanced illness receiving palliative care: Study 1 included a single-dose, double-blind, placebo-controlled period, whereas Study 2 included a 14-day multiple dose, double-blind, placebo-controlled period [see Clinical Studies]. The majority of patients had a primary diagnosis of incurable cancer; other primary diagnoses included end-stage COPD/emphysema, cardiovascular disease/heart failure, Alzheimer's disease/dementia, HIV/AIDS, or other advanced illnesses. Patients were receiving opioid therapy (median daily baseline oral morphine equivalent dose = 172 mg), and had opioid-induced constipation (either < 3 bowel movements in the preceding week or no bowel movement for 2 days). Both the methylnaltrexone bromide and placebo patients were on a stable laxative regimen for at least 3 days prior to study entry and continued on their regimen throughout the study.

The most common ( ≥ 5%) adverse reactions in patients receiving RELISTOR are shown in Table 2 below.

Table 2 : Adverse Reactions from all Doses in Double-Blind, Placebo-Controlled Clinical Studies of RELISTOR in Adult Patients with Opioid-Induced Constipation and Advanced Illness*

Adverse Reaction RELISTOR
N = 165
N = 123
Abdominal Pain 47 (28.5%) 12 (9.8%)
Flatulence 22 (13.3%) 7 (5.7%)
Nausea 19 (11.5%) 6 (4.9%)
Dizziness 12 (7.3%) 3 (2.4%)
Diarrhea 9 (5.5%) 3 (2.4%)
Hyperhidrosis 11 (6.7%) 8 (6.5%)
* Doses: 0.075, 0.15, and 0.30 mg/kg/dose

The rates of discontinuation due to adverse events during the double-blind placebo controlled clinical trials (Study 1 and Study 2) were comparable between RELISTOR (1.2%) and placebo (2.4%).

Postmarketing Experience

The following additional adverse reactions have been identified during post-approval use of RELISTOR. Because they are reported voluntarily from a population of unknown size, estimates of frequency cannot be made. These events have been chosen for inclusion due to either their seriousness, frequency of reporting or causal connection to RELISTOR, or a combination of these factors.


Perforation, cramping, vomiting

General Disorders and Administrative Site

Disorders Diaphoresis, flushing, malaise, pain. Cases of opioid withdrawal have been reported.

Read the Relistor (methylnaltrexone bromide) Side Effects Center for a complete guide to possible side effects


Drugs Metabolized by Cytochrome P450 Isozymes

In healthy subjects, a subcutaneous dose of 0.30 mg/kg of methylnaltrexone did not significantly affect the metabolism of dextromethorphan, a CYP2D6 substrate.

In vitro methylnaltrexone did not significantly inhibit or induce the activity of cytochrome P450 (CYP) isozymes CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, or CYP3A4.

In vitro, methylnaltrexone did not induce the enzymatic activity of CYP2E1.

Drugs Renally Excreted

Methylnaltrexone is actively secreted in the kidney. The potential of drug interactions between methylnaltrexone bromide and other drugs that are inhibitors of transporters in the kidney has not been fully investigated [see Pharmacokinetics].


Cimetidine given 400 mg three times daily did not significantly affect the systemic exposure to methylnaltrexone. The effect of a higher cimetidine dose (e.g., 800 mg) on the systemic exposure of methylnaltrexone has not been evaluated.

Last reviewed on RxList: 9/19/2013
This monograph has been modified to include the generic and brand name in many instances.

Side Effects

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You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.

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