A 32-year-old woman with community-acquired pneumonia is prescribed a macrolide antibiotic. Which macrolide is the most common cause of gastrointestinal side effects due to its prokinetic activity on the stomach?

Explanation

## Macrolide-Induced Gastrointestinal Effects **Key Point:** Erythromycin is the most common cause of GI side effects among macrolides because it acts as a motilin receptor agonist, stimulating gastric contractions and causing nausea, vomiting, and abdominal cramping. ### Mechanism of GI Toxicity Erythromycin binds to motilin receptors on gastric smooth muscle, increasing gastric motility and accelerating gastric emptying. This prokinetic effect, while sometimes therapeutically exploited, frequently causes adverse GI symptoms. ### Comparative GI Side Effect Profile | Macrolide | Motilin Activity | GI Side Effects | Frequency | Clinical Notes | |-----------|------------------|-----------------|-----------|----------------| | **Erythromycin** | **High** | **Nausea, vomiting, cramping** | **Most common** | Dose-dependent; limits tolerability | | Azithromycin | Low | Mild GI upset | Least common | Better tolerated; preferred in many settings | | Clarithromycin | Moderate | Nausea, diarrhea | Moderate | Less prokinetic than erythromycin | | Roxithromycin | Low | Minimal | Rare | Minimal motilin activity | **High-Yield:** Erythromycin's prokinetic effect is so pronounced that it is sometimes used therapeutically as a gastric prokinetic agent at lower doses (250 mg TID), but at therapeutic antibiotic doses (500 mg QID), GI side effects are nearly universal. ### Clinical Manifestations 1. **Nausea and vomiting** — occur in 10–30% of patients 2. **Abdominal cramping** — due to increased gastric contractions 3. **Diarrhea** — from altered GI motility and bacterial overgrowth 4. **Dose-dependent** — higher doses cause more severe symptoms 5. **Tachyphylaxis** — tolerance may develop with prolonged use **Clinical Pearl:** Azithromycin is preferred in patients with baseline GI symptoms or those intolerant to erythromycin because it has minimal motilin receptor activity and causes fewer GI side effects. **Warning:** Do not confuse erythromycin's prokinetic effect with its antibiotic action — the GI symptoms are a direct consequence of motilin agonism, not infection or Clostridioides difficile colitis (though macrolides can increase C. difficile risk). **Mnemonic:** **ERY-MOTILITY** — Erythromycin causes motilin-mediated GI motility disturbances.