Methylnaltrexone: What It Is, How It Works, and What You Need to Know

When you're taking opioids for chronic pain, constipation often comes along as an unwanted side effect. That’s where methylnaltrexone, a peripherally acting mu-opioid receptor antagonist used to treat opioid-induced constipation without interfering with pain relief. Also known as Relistor, it works by blocking opioid effects in the gut while leaving pain control untouched. Unlike regular laxatives, methylnaltrexone doesn’t just stimulate bowel movements—it targets the root cause: opioids binding to receptors in the intestines.

This medication is mainly used in people on long-term opioid therapy, especially those in palliative care, a medical approach focused on improving quality of life for patients with serious illnesses. It’s not for occasional constipation. If you’re on fentanyl patches, oxycodone, or morphine and your bowels have slowed down despite trying fiber, stool softeners, or stimulant laxatives, methylnaltrexone might be the next step. It’s given as a subcutaneous injection, usually once every other day, and works within 30 minutes to 4 hours for many users.

It’s not a cure-all, and it’s not for everyone. People with bowel obstructions or severe abdominal problems shouldn’t use it. Side effects like dizziness, nausea, or sweating are possible, but most tolerate it well. It’s often compared to naloxone derivatives, compounds designed to reverse opioid effects, but with key differences in how and where they act in the body. While naloxone can reverse pain relief if it crosses into the brain, methylnaltrexone is engineered to stay outside the central nervous system—so your pain stays managed, but your gut gets relief.

What you’ll find in the posts below are real-world comparisons, patient experiences, and practical advice on managing opioid-related side effects. You’ll see how methylnaltrexone stacks up against other treatments, what doctors look for before prescribing it, and how it fits into broader care plans for chronic pain and end-of-life care. No fluff. Just clear, actionable info from people who’ve been there.