Drug Interactions
🟢 Lite — Quick Review (1h–1d)
Drug Interactions — Key Facts for NEET PG
Pharmacokinetic Interactions:
- Absorption: Antacids (Mg/Al) ↓ absorption of Fluoroquinolones, Iron, Tetracyclines — space 2h apart
- Protein binding: Warfarin displaced by NSAIDs, Sulfonamides → increased free warfarin → bleeding risk
- Metabolism: Cimetidine inhibits CYP450; Phenytoin, Rifampicin induce CYP450
- Excretion: Probenecid blocks renal secretion of Methotrexate, Penicillins
Pharmacodynamic Interactions:
- Synergism: Warfarin + Aspirin → ↑ bleeding risk
- Antagonism: Naloxone reverses opioid effects
- Potentiation: ACE inhibitors + Potassium-sparing diuretics → hyperkalemia
High-yield point: Know the classic CYP450 inducers (Rifampicin, Phenytoin, Carbamazepine) and inhibitors (Cimetidine, Ketoconazole, Erythromycin)
🟡 Standard — Regular Study (2d–2mo)
Drug Interactions — NEET PG Study Guide
Mechanisms of Interaction:
| Type | Mechanism | Example |
|---|---|---|
| PK — Absorption | Chelation, pH change | Tetracycline + Iron |
| PK — Protein binding | Displacement from albumin | Warfarin + NSAID |
| PK — Metabolism | CYP450 induction/inhibition | Rifampicin + OCP |
| PK — Excretion | Renal tubular secretion | Probenecid + Methotrexate |
| PD — Synergism | Additive/amplified effect | Ethanol + Diazepam |
| PD — Antagonism | Opposing effects | Flumazenil + BDZ |
Cytochrome P450 System (High-Yield):
- CYP3A4: Most drugs metabolized; inhibited by Erythromycin, Ketoconazole
- CYP2D6: Beta-blockers, antidepressants; inhibited by Quinidine
- CYP1A2: Theophylline, Caffeine; induced by Smoking
Study strategy: Make a table of inducers vs inhibitors with drug examples — frequently asked in NEET PG
🔴 Extended — Deep Study (3mo+)
Drug Interactions — Comprehensive NEET PG Notes
Detailed Classification:
1. Absorption Interactions:
- Chelation: Fluoroquinolones + multivalent cations (Fe, Zn, Ca, Mg) — form insoluble complexes
- Gut motility: Anticholinergics ↓ Metoclopramide absorption; Prokinetics ↑ other drug absorption
- pH effect: H2 blockers, PPIs affect absorption of drugs needing acidic pH (Ketoconazole, Iron)
2. Metabolism-Based Interactions:
| Inducer | Substrate | Clinical Effect |
|---|---|---|
| Rifampicin | OCP, Warfarin | Treatment failure, ↓ INR |
| Phenytoin | OCP, Warfarin | Treatment failure |
| Carbamazepine | OCP, many AEDs | Treatment failure |
| Inhibitor | Substrate | Clinical Effect |
|---|---|---|
| Cimetidine | Warfarin, Phenytoin | ↑ levels, toxicity |
| Erythromycin | Theophylline, Statins | ↑ levels, toxicity |
| Ketoconazole | Many drugs | ↑ levels |
3. Excretion Interactions:
- Urinary alkalinization: ↑ excretion of acidic drugs (Salicylates, Barbiturates)
- Urinary acidification: ↑ excretion of basic drugs (Amphetamines, TCA)
- Probenecid: Competitively inhibits renal secretion of Methotrexate, Penicillins, Cephalosporins
4. Pharmacodynamic — Additive/Synergistic:
- CNS depressants: Benzodiazepines + Opioids + Alcohol → respiratory depression (avoid)
- Anticholinergics + Antihistamines: Additive anticholinergic effects (dry mouth, blur, urinary retention)
- ACEi + K+ supplements: Life-threatening hyperkalemia
Clinical Scenarios Frequently Asked:
- Warfarin + Metronidazole → ↑ INR (CYP2C9 inhibition)
- Theophylline + Erythromycin → Theophylline toxicity
- Methotrexate + Probenecid → Methotrexate toxicity
- OCP + Rifampicin → Contraceptive failure
Practice: Attempt previous year questions on drug interactions — this is a high-yield area for NEET PG
Content adapted based on your selected roadmap duration. Switch tiers using the selector above.