Read the Medication Guide and Instructions for Use provided by your pharmacist before you start using methylnaltrexone and each time you get a refill. Ask your health care professional how to prepare and use this medication. If you have any questions, ask your doctor or pharmacist.

Inject this medication under the skin as directed by your doctor, usually once daily on a set schedule or once every other day as needed. Follow your doctor's instructions carefully. Do not use more than once in 24 hours. The injection is given in the upper arm, abdomen, or thigh. Do not inject into a muscle or vein. The dosage is based on your weight, medical condition, and response to treatment.

Before using, check this product for particles or discoloration. If either is present, do not use the liquid. Before injecting each dose, clean the skin you are going to inject into (the injection site) with rubbing alcohol. Change the injection site each time to lessen injury under the skin. To lessen bruising, do not rub the injection site after a shot. Learn how to store and discard medical supplies safely.

Ask your doctor if you should keep using your usual medications for constipation. This medication is used only for constipation caused by opioids. Stop using methylnaltrexone when you stop using opioid pain medication unless otherwise directed by your doctor.

Make sure you are close to a bathroom after using this medication. This medication works quickly. Most people have a bowel movement within a few minutes to a few hours after using this medication.

Tell your doctor if your condition lasts or gets worse.