U01.05.034 Drug reactions—gastrointestinal

Understanding gastrointestinal drug reactions is vital for USMLE Step 1 preparation. Below is a detailed guide categorized by reaction type, causal agents, and clinical notes.

Types of Drug Reactions and Causal Agents

1. Acute Cholestatic Hepatitis and Jaundice

Causal Agents: Macrolides (e.g., erythromycin)
Notes: Often associated with macrolides’ hepatic effects.

2. Diarrhea

Causal Agents:
- Antidiabetic Agents: Acarbose, metformin, pramlintide
- Others: Acamprosate, colchicine, cholinesterase inhibitors, lipid-lowering agents (e.g., ezetimibe, orlistat), macrolides (e.g., erythromycin), SSRIs, chemotherapy agents (e.g., irinotecan)
Notes: A broad range of medications cause diarrhea due to effects on gut motility or secretion.

3. Focal to Massive Hepatic Necrosis

Causal Agents:
- Halothane
- Amanita phalloides (death cap mushroom)
- Valproic acid
- Acetaminophen
Mnemonic: Liver “HAVAc” (Halothane, Amanita phalloides, Valproic acid, Acetaminophen)
Notes: Mechanisms often involve direct hepatocyte damage.

4. Hepatitis

Causal Agents: Rifampin, isoniazid, pyrazinamide, statins, fibrates
Notes: Particularly notable with antitubercular drugs and lipid-lowering agents.

5. Pancreatitis

Causal Agents:
- Didanosine
- Corticosteroids
- Alcohol
- Valproic acid
- Azathioprine
- Diuretics (e.g., furosemide, hydrochlorothiazide)
Mnemonic: Drugs Causing A Violent Abdominal Distress

6. Pill-Induced Esophagitis

Causal Agents: Bisphosphonates, ferrous sulfate, NSAIDs, potassium chloride, tetracyclines
Notes: Minimized with upright posture and sufficient water ingestion.

7. Pseudomembranous Colitis

Causal Agents: Ampicillin, cephalosporins, clindamycin, fluoroquinolones, proton pump inhibitors (PPIs)
Notes: Antibiotic use predisposes patients to C. difficile overgrowth.

Summary Table: Gastrointestinal Drug Reactions

Reaction Type	Causal Agents	Notes
Acute cholestatic hepatitis	Macrolides (e.g., erythromycin)	Liver toxicity
Diarrhea	Acamprosate, antidiabetic agents, lipid-lowering agents, SSRIs, chemotherapy, macrolides	Secretory or motility effects
Hepatic necrosis	Halothane, Amanita phalloides, valproic acid, acetaminophen	Mnemonic: Liver “HAVAc”
Hepatitis	Rifampin, isoniazid, pyrazinamide, statins, fibrates	Often drug-induced hepatotoxicity
Pancreatitis	Didanosine, corticosteroids, alcohol, valproic acid, azathioprine, diuretics	Mnemonic: Drugs Causing A Violent Abdominal Distress
Pill-induced esophagitis	Bisphosphonates, ferrous sulfate, NSAIDs, potassium chloride, tetracyclines	Preventable with upright posture & water intake
Pseudomembranous colitis	Ampicillin, cephalosporins, clindamycin, fluoroquinolones, PPIs	Related to C. difficile superinfection

Points to Remember

Key Mnemonics:
- Liver “HAVAc” (Hepatic necrosis)
- Drugs Causing A Violent Abdominal Distress (Pancreatitis)
High-Yield Notes:
- Pseudomembranous colitis results from antibiotic overuse, enabling C. difficile.
- For pill-induced esophagitis, prevention is simple: stay upright and hydrate after ingestion.
- Be cautious of macrolides as they cause multiple reactions (cholestatic hepatitis, diarrhea).

Tips for Exam Success

Familiarize yourself with mnemonics for rapid recall.
Link drug categories to their typical adverse effects (e.g., lipid-lowering agents with hepatitis).
Understand prevention strategies for reactions like esophagitis.

Bibliography

First Aid for the USMLE Step 1 2024.
Goodman & Gilman’s The Pharmacological Basis of Therapeutics.
UpToDate: Drug-Induced Liver Injury.