Methylnaltrexone belongs to a group of medications known as opioid receptor antagonists. It is used to treat constipation caused by prescription pain medications called opioids (e.g., codeine, morphine, hydromorphone) for people who are receiving palliative care. If laxatives are not working, methylnaltrexone is used along with laxatives to help produce a bowel movement.

It works by preventing opioid medications from binding to certain receptors in the intestinal tract. It does not reduce the pain-relieving effects of opioid pain medications. For most people, methylnaltrexone usually works within 30 minutes, but it may take longer.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are being given this medication, speak to your doctor. Do not stop using this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to use this medication if their doctor has not prescribed it.

The usual recommended dose of methylnaltrexone is based on body weight:

• 6 mg for people who weigh 33 kg to less than 38 kg
• 8 mg for people who weigh 38 kg to less than 62 kg
• 12 mg for people who weight 62 kg to 114 kg
• 18 mg for people who weigh 115 kg to 126 kg

For people who weigh less than 33 kg or more than 126 kg, your doctor will calculate the dose of this medication that is appropriate for you.

Methylnaltrexone is injected once every other day under the skin of the upper arm, abdomen, or thigh.

It is important to use this medication exactly as prescribed by your doctor. If you (or a family member or friend) are injecting this medication, a health care professional will show you how to inject the medication properly. Read the instructions provided in the package insert carefully. If you are not sure how to use this medication, contact your doctor or pharmacist.

It is recommended that you use a different site for each injection. Do not inject into skin that is tender, red, bruised, hard, or has stretch marks or scars. If you are giving the injection to yourself, do not inject into the upper arm. You should be sitting or lying down when you receive this medication. Do not stand up quickly after the injection as you may feel dizzy.

If you do not notice improvement within a week, contact your doctor.

Many things can affect the dose of a medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are using the medication without consulting your doctor.

Store this medication at room temperature, protect it from light, and keep it out of the reach of children. Do not allow it to freeze.

Each mL of sterile, clear and colourless to pale yellow aqueous solution for injection contains methylnaltrexone bromide 20 mg. Each vial contains 12 mg of methylnaltrexone bromide in 0.6 mL of sterile water for injection. Nonmedicinal ingredients: edetate calcium disodium, glycine hydrochloride, sodium chloride, and sterile water for injection. The solution is isotonic and the pH is adjusted during manufacture with hydrochloric acid and/or sodium hydroxide to approximately 3.4.

Do not use methylnaltrexone if you:

• are allergic to methylnaltrexone or to any of the ingredients of this medication
• have or may have a blockage of the gastrointestinal tract or abdominal pain that requires immediate surgery

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent. The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• abdominal pain
• dizziness
• drowsiness
• fatigue
• gas
• nausea
• pain at the place of injection
• restlessness
• sweating
• vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• diarrhea

Stop taking the medication and seek immediate medical attention if any of the following occur:

• severe, worsening, or persistent abdominal symptoms

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Gastrointestinal perforation: People with advanced illness who use methylnaltrexone injection may be at increased risk of gastrointestinal perforation (a hole that forms in the wall of the gastrointestinal tract, such as through the stomach or intestines). People are especially at risk if they also have a weakened gastrointestinal wall due to conditions such as cancer, intestinal cancer, or gastrointestinal cancer. If you experience severe, worsening, or persistent abdominal pain that's intensified by movement, nausea and vomiting, or accompanied by fever or chills, seek immediate medical attention. These could be symptoms of gastrointestinal perforation, which is a medical emergency.

Kidney function: People with severely reduced kidney function require lower doses of this medication. If you have kidney disease or decreased kidney function, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Liver function: Methylnaltrexone is not recommended for people who have severely reduced liver function.

Severe diarrhea: This medication may cause severe diarrhea. If you experience this, stop using methylnaltrexone and contact your doctor as soon as possible.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while using this medication, contact your doctor immediately.

Breast-feeding: It is not known if methylnaltrexone passes into breast milk. If you are a breast-feeding mother and are using this medication, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of using this medication have not been established for children.

There may be an interaction between methylnaltrexone and any of the following:

• naloxegol
• naloxone
• naltrexone

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.