Which feature best distinguishes Type I (pharmacokinetic) drug interactions from Type II (pharmacodynamic) drug interactions?

## Classification of Drug Interactions ### Type I (Pharmacokinetic) Interactions **Key Point:** These involve changes in drug absorption, distribution, metabolism, or elimination, leading to altered plasma concentrations of one or both drugs. 1. **Absorption:** Antacids reduce fluoroquinolone absorption 2. **Distribution:** Displacement from protein binding (e.g., warfarin + NSAIDs) 3. **Metabolism:** CYP450 enzyme induction (e.g., rifampicin increases warfarin metabolism) or inhibition (e.g., ketoconazole inhibits CYP3A4) 4. **Elimination:** Reduced renal clearance (e.g., NSAIDs reduce ACE inhibitor elimination) ### Type II (Pharmacodynamic) Interactions **Key Point:** These occur when two drugs act on the same physiological system or receptor, producing additive, synergistic, or antagonistic effects WITHOUT altering each other's plasma concentration. 1. **Additive effects:** Two CNS depressants (alcohol + benzodiazepines) 2. **Synergistic effects:** ACE inhibitor + potassium-sparing diuretic (both increase K⁺) 3. **Antagonistic effects:** Beta-blocker + salbutamol (opposing effects on β-adrenergic receptors) ### Discriminating Features | Feature | Type I (Pharmacokinetic) | Type II (Pharmacodynamic) | | --- | --- | --- | | **Mechanism** | Altered drug levels (absorption, metabolism, elimination) | Combined effects at same target/receptor | | **Plasma concentration** | Changed | Unchanged | | **Onset** | May be delayed (enzyme induction takes days) | Usually rapid | | **Reversibility** | Reversible after drug withdrawal | Reversible after drug withdrawal | | **Example** | Rifampicin ↓ warfarin levels | Warfarin + aspirin → ↑ bleeding risk | **High-Yield:** The **best discriminator** is the site of interaction: Type I affects drug handling (ADME); Type II affects drug action (receptor/site level). **Clinical Pearl:** A patient on warfarin started on rifampicin (Type I: enzyme induction) will need warfarin dose increase to maintain INR. A patient on warfarin given aspirin (Type II: both inhibit coagulation) will have increased bleeding without any change in warfarin level.