## PPI Adverse Effects and Drug Interactions **Key Point:** PPIs inhibit CYP2C19 and CYP3A4, which **decreases** (not increases) metabolism of warfarin and other CYP-dependent drugs, leading to increased anticoagulant effect, not reduced. ### Major PPI-Related Adverse Effects | Adverse Effect | Mechanism | Clinical Significance | |---|---|---| | **C. difficile infection** | ↓ Gastric acidity → ↑ bacterial colonization | Increased diarrhea risk | | **Hypomagnesemia** | ↓ Mg²⁺ absorption in acidic environment | Arrhythmias, tetany | | **Hypocalcemia** | ↓ Ca²⁺ absorption (requires acidic pH) | Osteoporosis, fractures | | **Vitamin B₁₂ deficiency** | ↓ Intrinsic factor; ↓ absorption | Megaloblastic anemia (rare) | | **Iron deficiency** | ↓ Fe²⁺ absorption (needs acidic pH) | Anemia | ### PPI Drug Interactions **High-Yield:** PPIs are **CYP2C19 and CYP3A4 INHIBITORS**, not inducers. | Drug | Interaction | Outcome | |---|---|---| | **Clopidogrel** | CYP2C19 inhibition → ↓ activation | ↓ Antiplatelet effect (DANGEROUS) | | **Warfarin** | CYP2C19 inhibition → ↓ metabolism | ↑ INR, bleeding risk | | **Phenytoin** | CYP inhibition → ↓ metabolism | ↑ Phenytoin levels | | **Digoxin** | P-glycoprotein inhibition | ↑ Digoxin levels | **Clinical Pearl:** The clopidogrel–PPI interaction is clinically significant in post-PCI patients and is a major concern in cardiology. **Warning:** Students often confuse PPI metabolism effects. Remember: PPIs **inhibit** CYP enzymes, so they **decrease** metabolism of substrate drugs, leading to **increased** drug levels and effects — not decreased.
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