Pharmacokinetic and pharmacodynamic drug interactions are central to safe medication use and essential knowledge for B. Pharm students. This set of MCQs focuses on mechanisms—absorption, distribution (protein binding), metabolism (CYP450 enzymes, enzyme induction and inhibition), and excretion (renal transporters, P‑glycoprotein)—and pharmacodynamic concepts such as synergism, antagonism, and adverse effects like QT prolongation or serotonin syndrome. Emphasis is placed on clinical relevance, examples (warfarin, digoxin, statins, opioids), genetic variability, and strategies including dose adjustment and therapeutic drug monitoring. These questions will deepen understanding of interaction types, prediction, prevention, and management in pharmacy practice. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which change best describes a pharmacokinetic interaction?
- Two drugs producing enhanced therapeutic effect at receptor level
- Altered drug plasma concentration due to enzyme induction
- Competitive antagonism at the same receptor
- Opposing physiological effects without changing concentrations
Correct Answer: Altered drug plasma concentration due to enzyme induction
Q2. Which mechanism commonly causes increased plasma levels of a drug when co-administered with ketoconazole?
- Induction of CYP3A4
- Inhibition of CYP3A4
- Activation of P‑glycoprotein efflux
- Enhanced renal tubular secretion
Correct Answer: Inhibition of CYP3A4
Q3. A drug displaces another from plasma albumin binding. This interaction primarily affects which parameter?
- Volume of distribution and free (unbound) fraction
- Intrinsic hepatic clearance only
- Gastric absorption rate exclusively
- Renal glomerular filtration rate
Correct Answer: Volume of distribution and free (unbound) fraction
Q4. Which is a classic example of a pharmacodynamic antagonistic interaction?
- Warfarin and amiodarone increasing INR
- Beta‑blocker reducing the effect of albuterol
- Rifampin decreasing oral contraceptive levels
- Grapefruit juice increasing statin concentration
Correct Answer: Beta‑blocker reducing the effect of albuterol
Q5. Rifampin reduces plasma concentrations of many drugs by what primary mechanism?
- Inhibition of renal secretion
- Induction of CYP enzymes
- Competitive displacement from receptors
- Inhibition of intestinal absorption
Correct Answer: Induction of CYP enzymes
Q6. Which transporter is most associated with drug efflux in the intestine and drug interactions?
- Organic anion transporting polypeptide (OATP)
- P‑glycoprotein (P‑gp)
- Glucose transporter (GLUT)
- Monoamine transporter (MAT)
Correct Answer: P‑glycoprotein (P‑gp)
Q7. Combining two drugs with additive anticoagulant effects (e.g., warfarin and aspirin) is an example of:
- Pharmacokinetic induction
- Pharmacodynamic synergism or additive effect
- Protein binding displacement only
- Enhanced renal excretion
Correct Answer: Pharmacodynamic synergism or additive effect
Q8. Which CYP isoform is most important for metabolizing codeine to morphine and is subject to genetic polymorphism?
- CYP2C9
- CYP3A4
- CYP2D6
- CYP1A2
Correct Answer: CYP2D6
Q9. A patient on digoxin is prescribed verapamil. Which interaction is most likely?
- Verapamil reduces digoxin absorption by chelation
- Verapamil increases digoxin plasma levels via P‑gp inhibition
- Verapamil induces renal clearance of digoxin
- No interaction expected clinically
Correct Answer: Verapamil increases digoxin plasma levels via P‑gp inhibition
Q10. Which food–drug interaction is correctly matched?
- Grapefruit juice — induces CYP3A4 increasing drug clearance
- Leafy greens — decrease warfarin effect due to vitamin K
- High‑protein meals — universally decrease drug absorption
- Dairy products — induce tetracycline metabolism
Correct Answer: Leafy greens — decrease warfarin effect due to vitamin K
Q11. Which describes a mechanism of competitive enzyme inhibition?
- Inhibitor binds to allosteric site reducing enzyme Vmax
- Inhibitor binds to active site increasing Km without changing Vmax
- Inhibitor irreversibly destroys enzyme, lowering Vmax
- Inhibitor enhances enzyme synthesis
Correct Answer: Inhibitor binds to active site increasing Km without changing Vmax
Q12. Co‑administration of SSRIs with MAO inhibitors can lead to:
- QT interval shortening
- Serotonin syndrome due to pharmacodynamic interaction
- Accelerated CYP450 metabolism reducing SSRI levels
- Enhanced renal clearance of MAO inhibitors
Correct Answer: Serotonin syndrome due to pharmacodynamic interaction
Q13. Which interaction increases risk of torsades de pointes (QT prolongation)?
- Combining macrolide antibiotics with statins
- Combining two drugs that both prolong QT interval (e.g., sotalol + erythromycin)
- Combining ACE inhibitors with ARBs
- Combining beta‑lactam antibiotics with aminoglycosides
Correct Answer: Combining two drugs that both prolong QT interval (e.g., sotalol + erythromycin)
Q14. Which statement about first‑pass metabolism interactions is correct?
- Inhibition of intestinal CYP3A4 increases oral bioavailability of some drugs
- Induction of intestinal enzymes reduces intravenous drug exposure
- First‑pass effects only occur in the kidneys
- Protein binding changes have no impact on first‑pass metabolism
Correct Answer: Inhibition of intestinal CYP3A4 increases oral bioavailability of some drugs
Q15. A prodrug requiring bioactivation by CYP2C19 will have reduced effect if given with which agent?
- Omeprazole, a CYP2C19 inhibitor
- Carbamazepine, a CYP3A4 inhibitor
- Phenobarbital, a CYP2C19 inducer
- Rifampin, a P‑gp inhibitor
Correct Answer: Omeprazole, a CYP2C19 inhibitor
Q16. Therapeutic drug monitoring (TDM) is especially important when:
- Drugs have wide therapeutic index and predictable effects
- Drugs have narrow therapeutic index and variable pharmacokinetics
- All drugs metabolized solely by CYP3A4
- The drug is only administered topically
Correct Answer: Drugs have narrow therapeutic index and variable pharmacokinetics
Q17. Which best describes a mixed type of enzyme inhibition?
- Inhibitor binds only to the active site and competes with substrate
- Inhibitor binds to a different site and affects both Km and Vmax
- Inhibitor increases enzyme expression
- Inhibitor only lowers Km without changing Vmax
Correct Answer: Inhibitor binds to a different site and affects both Km and Vmax
Q18. Co‑prescribing aspirin with methotrexate can increase methotrexate toxicity primarily by:
- Inhibiting CYP2D6 mediated metabolism
- Reducing renal clearance via competition for tubular secretion
- Enhancing hepatic glucuronidation
- Inducing methotrexate protein binding
Correct Answer: Reducing renal clearance via competition for tubular secretion
Q19. A strong CYP3A4 inducer will most likely have what effect on midazolam plasma levels?
- Increase midazolam levels causing sedation
- Decrease midazolam levels reducing its effect
- No change because midazolam is renally excreted unchanged
- Convert midazolam to an active prodrug
Correct Answer: Decrease midazolam levels reducing its effect
Q20. Which interaction is mediated primarily by alteration of gastric pH affecting absorption?
- Omeprazole reducing absorption of ketoconazole
- Rifampin inducing warfarin metabolism
- Loop diuretics increasing lithium excretion
- Cimetidine displacing phenytoin from albumin
Correct Answer: Omeprazole reducing absorption of ketoconazole
Q21. Which pairing is an example of enzyme inhibition causing clinically significant interaction?
- Rifampin and oral contraceptives
- Fluconazole and cyclosporine
- Phenobarbital and lamotrigine
- Carbamazepine and simvastatin
Correct Answer: Fluconazole and cyclosporine
Q22. Which term describes when one drug enhances the effect of another without having that effect alone?
- Potentiation
- Antagonism
- Competitive inhibition
- First‑pass effect
Correct Answer: Potentiation
Q23. Which interaction increases bleeding risk in a patient on warfarin?
- Rifampin co‑administration
- Trimethoprim‑sulfamethoxazole inhibiting CYPs and displacing protein binding
- Phenytoin inducing warfarin metabolism
- High vitamin K diet
Correct Answer: Trimethoprim‑sulfamethoxazole inhibiting CYPs and displacing protein binding
Q24. When a drug reduces biliary excretion of another drug by inhibiting hepatic transporters, this is an interaction affecting:
- Absorption only
- Distribution only
- Elimination (excretion)
- Receptor sensitivity
Correct Answer: Elimination (excretion)
Q25. Which describes a time‑dependent interaction where effects may persist after stopping the interacting drug?
- Competitive reversible inhibition
- Enzyme induction due to increased enzyme synthesis
- Protein binding displacement
- Immediate transporter blockade
Correct Answer: Enzyme induction due to increased enzyme synthesis
Q26. St. John’s wort induces CYP3A4 and P‑gp. Which clinical consequence is likely when combined with oral contraceptives?
- Increased contraceptive levels causing toxicity
- Decreased contraceptive effectiveness and risk of pregnancy
- No significant change in contraceptive effect
- Enhanced absorption via gastric pH changes
Correct Answer: Decreased contraceptive effectiveness and risk of pregnancy
Q27. A drug that is a mechanism-based (suicide) inhibitor of a CYP enzyme will:
- Cause reversible competitive inhibition
- Irreversibly inactivate the enzyme leading to prolonged inhibition
- Only affect renal clearance pathways
- Always increase Vmax of the enzyme
Correct Answer: Irreversibly inactivate the enzyme leading to prolonged inhibition
Q28. In polypharmacy, monitoring which parameter helps detect pharmacokinetic interactions affecting renal elimination?
- Serum albumin only
- Serum creatinine and drug plasma concentrations
- ECG exclusively
- Arterial blood gases
Correct Answer: Serum creatinine and drug plasma concentrations
Q29. Which clinical strategy can reduce the risk of significant drug interactions?
- Avoid therapeutic drug monitoring for narrow therapeutic index drugs
- Use lowest effective dose, review patient meds, and monitor levels/side effects
- Always add more drugs to counteract interactions
- Ignore OTC and herbal medicines in medication history
Correct Answer: Use lowest effective dose, review patient meds, and monitor levels/side effects
Q30. Combining an opioid with a benzodiazepine is dangerous mainly because of:
- Enhanced hepatic enzyme induction
- Pharmacodynamic additive respiratory depression
- Competitive displacement from albumin increasing opioid clearance
- Reduced CNS penetration of both agents
Correct Answer: Pharmacodynamic additive respiratory depression
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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