Learning Objective
By the end of this section, students should be able to identify common gastrointestinal adverse effects caused by medications, the responsible drugs, and key clinical features, which are high-yield for USMLE Step 1 pharmacology.
Drug-Induced Gastrointestinal Reactions
| Reaction | Causal Agents | Notes / Clinical Features |
|---|---|---|
| Acute cholestatic hepatitis/jaundice | Macrolides (e.g., erythromycin) | Typically mild and reversible; presents with elevated bilirubin and liver enzymes |
| Constipation | Antimuscarinics (e.g., atropine), Antipsychotics, Opioids, Non-dihydropyridine CCBs, Ranolazine, Amiodarone, Aluminum hydroxide, Loperamide, 5HT3 receptor antagonists (ondansetron), Vincristine | Common side effect: may require laxatives or dietary fiber |
| Diarrhea | Acamprosate, Antidiabetic agents (acarbose, metformin, pramlintide), Colchicine, Cholinesterase inhibitors, Lipid-lowering agents (ezetimibe, orlistat), Macrolides (erythromycin), SSRIs, Chemotherapy (irinotecan) | Can range from mild to severe; monitor hydration and electrolytes |
| Focal to massive hepatic necrosis | Amanita phalloides (death cap mushroom), Valproate, Acetaminophen | Life-threatening; presents with markedly elevated AST/ALT, jaundice, coagulopathy; supportive care and antidotes (N-acetylcysteine for acetaminophen) |
| Hepatitis | Rifampin, Isoniazid, Pyrazinamide, Statins, Fibrates | Usually asymptomatic or mild; monitor LFTs during therapy |
| Pancreatitis | Diuretics (furosemide, HCTZ), Glucocorticoids, Alcohol, Valproate, Azathioprine | Presents with abdominal pain, nausea/vomiting, and elevated amylase/lipase |
| Medication-induced esophagitis | Potassium chloride, NSAIDs, Bisphosphonates, Ferrous sulfate, Tetracyclines | Usually, at sites of esophageal narrowing, prevention is achieved with an upright posture and adequate water |
| Pseudomembranous colitis | Ampicillin, Cephalosporins, Clindamycin, Fluoroquinolones, PPIs | Caused by C. difficile superinfection; presents with watery diarrhea, abdominal pain, and fever; treated with oral vancomycin or fidaxomicin |
USMLE Step 1 Pearls
- Macrolides → cholestatic hepatitis; usually mild.
- Opioids/antimuscarinics → constipation; monitor in elderly or immobilized patients.
- Acetaminophen → massive hepatic necrosis; antidote: N-acetylcysteine.
- Medications like bisphosphonates → pill esophagitis; prevent with upright posture.
- Antibiotics / PPIs → C. difficile; classic pseudomembranous colitis.
- Diuretics, valproate → pancreatitis; monitor lipase/amylase.
