A 48-year-old woman with cancer pain is being managed with opioid analgesics. She develops severe constipation despite laxative use. The pain management team decides to add a peripheral mu-opioid receptor antagonist. Which is the most common opioid-induced side effect for which peripheral antagonists are indicated?

## Most Common Opioid-Induced Side Effect Requiring Peripheral Antagonism **Key Point:** Opioid-induced constipation (OIC) is the most common and most troublesome opioid side effect, occurring in 40–80% of opioid-treated patients, and is the primary indication for peripheral mu-opioid receptor antagonists such as methylnaltrexone and naloxegol. ### Why OIC is the Most Common 1. **Mechanism of Opioid-Induced Constipation** - Mu-opioid receptors in the enteric nervous system (myenteric and submucosal plexuses) reduce acetylcholine release. - This decreases gastric motility, increases segmental contractions, and reduces propulsive peristalsis. - Mu receptors in the colon increase fluid absorption, further hardening stool. 2. **Incidence and Clinical Impact** - OIC occurs in 40–80% of patients on chronic opioid therapy (higher than nausea/vomiting at 20–30%). - Unlike tolerance to respiratory depression and analgesia, tolerance to constipation does NOT develop. - OIC is the leading reason for opioid dose reduction or discontinuation in cancer pain patients. 3. **Why Peripheral Antagonists Are Preferred** - Peripheral mu-opioid antagonists (methylnaltrexone, naloxegol, alvimopan) block opioid receptors in the GI tract without crossing the blood–brain barrier. - They reverse OIC without reducing analgesia or precipitating withdrawal. - Central antagonists (naloxone) would reverse all opioid effects, including analgesia. ### Comparison of Opioid Side Effects and Management | Side Effect | Incidence | Tolerance Develops | Management | |-------------|-----------|-------------------|-------------| | Constipation | 40–80% | No | Laxatives, peripheral antagonists | | Nausea/Vomiting | 20–30% | Yes (3–5 days) | Antiemetics, rotation | | Respiratory Depression | 5–10% | Yes | Naloxone (central antagonist) | | Urinary Retention | <5% | Varies | Catheterization, alpha-blockers | **High-Yield:** OIC is the MOST COMMON opioid side effect that does NOT show tolerance. This is why it requires proactive management in all opioid patients. **Mnemonic:** **OICC** = **O**pioid-**I**nduced **C**onstipation is **C**ommon (40–80% of patients). **Clinical Pearl:** Peripheral mu-opioid antagonists are contraindicated if there is suspected GI perforation or mechanical obstruction. Always rule out organic causes of constipation before attributing it to opioids. **Tip:** In the exam, when asked about "most common opioid side effect," think constipation. When asked about "peripheral antagonist," think OIC. These are linked concepts.