How Do Peripherally-Acting Mu-Opioid Receptor Antagonists Work?

Peripherally-acting mu-opioid receptor antagonists (PAMORAs) are medications to treat opioid-induced constipation. Constipation is one of the most common side effects of opioid drugs and PAMORAs prevent constipation without compromising their pain-relieving (analgesic) effects.

Opioid analgesic medications prevent pain by stimulating opioid receptors, which block the release of chemicals (neurotransmitters) that transmit pain signals. Opioid analgesics also stimulate opioid receptors in the gastrointestinal (GI) tract, which slows down the motility of the GI contents, resulting in constipation.

Opioid receptors are protein molecules on nerve cell (neuron) membranes in the central and peripheral nervous systems. Opioid receptors mediate the body’s response to most hormones and some of their functions include modulating pain, stress response, respiration, digestion, mood, and emotion.

PAMORAs are opioid antagonists, which bind to mu-opioid receptors in the gastrointestinal tract and prevent opioid agonists from stimulating them. Mu-opioid receptors are a major type of opioid receptors and most opioid analgesics work by binding to them.

Peripherally-acting mu-opioid receptor antagonists are administered orally as tablets or capsules, or as injections into the tissue under the skin (subcutaneous). PAMORAs are used to prevent constipation in adults in the following situations:

• Gastrointestinal recovery post bowel resection surgery
• Opioid-induced constipation in people with advanced illness who are receiving palliative care
• Opioid-induced constipation in chronic noncancer pain

Side effects of peripherally-acting mu-opioid receptor antagonists may include the following:

• Abdominal pain
• Flatulence
• Nausea
• Vomiting
• Dyspepsia (indigestion)
• Diarrhea
• Constipation
• Gastroenteritis (gastrointestinal inflammation)
• Hypokalemia (low potassium level in blood)
• Anemia
• Dizziness
• Headache
• Back pain
• Gastrointestinal cramping
• Gastrointestinal perforation
• Opioid withdrawal
• Hyperhidrosis (excessive sweating)
• Flushing
• Malaise
• Pain
• Bronchospasm
• Rash
• Urticaria (hives)
• Angioedema (swelling in the tissue under the skin or mucous membranes)

Information contained herein is not intended to cover all possible side effects, precautions, warnings, drug interactions, allergic reactions, or adverse effects. Check with your doctor or pharmacist to make sure these drugs do not cause any harm when you take them along with other medicines. Never stop taking your medication and never change your dose or frequency without consulting your doctor.

Generic and brand names of some peripherally-acting mu-opioid receptor antagonist drugs include:

• alvimopan
• Entereg
• methylnaltrexone
• Movantik
• naldemedine
• naloxegol
• Relistor
• Symproic