Combined effects of drugs describe how two or more drugs interact to alter therapeutic outcomes and adverse effects. For B.Pharm students, understanding drug interactions is essential: pharmacodynamic interactions (synergism, antagonism, potentiation) and pharmacokinetic interactions (absorption, distribution, metabolism, excretion) affect efficacy and safety. Key concepts include cytochrome P450 modulation, enzyme induction/inhibition, protein binding displacement, and changes in renal or hepatic clearance. Clinical significance covers polypharmacy, narrow therapeutic index drugs, and strategies to predict and manage interactions. Mastery of these principles improves dosing decisions, monitoring, and patient counselling. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What term describes two drugs producing a greater combined effect than the sum of their individual effects?
- Antagonism
- Additivity
- Synergism
- Competitive inhibition
Correct Answer: Synergism
Q2. Which mechanism is an example of a pharmacokinetic interaction?
- Two drugs acting on the same receptor
- One drug inducing CYP3A4, reducing another drug’s plasma level
- One drug blocking a receptor to reduce action of another
- Two drugs producing opposite physiological effects
Correct Answer: One drug inducing CYP3A4, reducing another drug’s plasma level
Q3. What is potentiation in drug interactions?
- Two drugs having identical mechanisms
- One drug increases the effect of another without having the effect itself
- Both drugs compete for the same receptor causing reduced effect
- Drugs causing opposite effects at different receptors
Correct Answer: One drug increases the effect of another without having the effect itself
Q4. Which change best describes a drug interaction that affects distribution?
- Altered gastric pH changing absorption
- Displacement from plasma proteins increasing free drug concentration
- Inhibition of renal tubular secretion
- Induction of hepatic metabolizing enzymes
Correct Answer: Displacement from plasma proteins increasing free drug concentration
Q5. Which interaction increases risk of bleeding when combined with warfarin?
- Drugs that induce CYP2C9
- Drugs that inhibit CYP2C9
- Drugs that increase vitamin K levels
- Drugs that enhance renal excretion
Correct Answer: Drugs that inhibit CYP2C9
Q6. A patient on digoxin experiences toxicity after starting spironolactone. Which mechanism explains this?
- Increased hepatic metabolism of digoxin
- Reduced renal clearance due to potassium sparing diuretic effect altering renal handling
- Protein binding displacement
- Induction of P-glycoprotein by spironolactone
Correct Answer: Reduced renal clearance due to potassium sparing diuretic effect altering renal handling
Q7. Which description fits competitive antagonism at a receptor?
- Irreversible receptor blockade reducing maximal response
- An antagonist that increases intrinsic activity of the agonist
- An antagonist that shifts agonist dose–response curve right without lowering maximal effect
- Two agonists producing additive effects at different receptors
Correct Answer: An antagonist that shifts agonist dose–response curve right without lowering maximal effect
Q8. Which enzyme family is most commonly involved in drug–drug metabolic interactions?
- Monoamine oxidases (MAO)
- UDP-glucuronosyltransferases (UGT)
- Cyclooxygenases (COX)
- Cytochrome P450s (CYP)
Correct Answer: Cytochrome P450s (CYP)
Q9. What is a major clinical risk of additive CNS depression when benzodiazepines are combined with opioids?
- Enhanced renal clearance
- Reduced risk of dependence
- Increased respiratory depression
- Decreased analgesic efficacy
Correct Answer: Increased respiratory depression
Q10. A drug that inhibits P‑glycoprotein will most likely cause which effect on a P‑gp substrate?
- Decreased intestinal absorption of the substrate
- Increased efflux from target cells
- Increased systemic exposure of the substrate
- Enhanced renal elimination of the substrate
Correct Answer: Increased systemic exposure of the substrate
Q11. Which term describes when two drugs produce a combined effect equal to the sum of their individual effects?
- Synergism
- Additivity
- Antagonism
- Potentiation
Correct Answer: Additivity
Q12. How can antacids alter the absorption of certain oral drugs?
- By inducing hepatic enzymes
- By chelating drugs or changing gastric pH reducing dissolution and absorption
- By increasing renal excretion of drugs
- By enhancing first‑pass metabolism
Correct Answer: By chelating drugs or changing gastric pH reducing dissolution and absorption
Q13. Combining a CYP450 inducer with an oral contraceptive may lead to what clinical outcome?
- Increased contraceptive efficacy
- Reduced plasma levels of contraceptive, risking unintended pregnancy
- No change in contraceptive effectiveness
- Increased risk of thromboembolism due to higher estrogen levels
Correct Answer: Reduced plasma levels of contraceptive, risking unintended pregnancy
Q14. Which interaction type is most likely when two drugs share the same elimination pathway and one slows that pathway?
- Pharmacodynamic synergism
- Pharmacokinetic competition leading to decreased clearance and increased levels
- Potentiation without pharmacokinetic change
- Competitive receptor antagonism
Correct Answer: Pharmacokinetic competition leading to decreased clearance and increased levels
Q15. A patient taking MAO inhibitors should avoid tyramine-rich foods because of risk of:
- Hypoglycemia
- Serotonin syndrome
- Hypertensive crisis due to excessive catecholamine release
- Prolonged QT interval
Correct Answer: Hypertensive crisis due to excessive catecholamine release
Q16. Which combination exemplifies pharmacodynamic antagonism?
- Beta‑agonist plus beta‑blocker
- Enzyme inducer plus substrate
- Two drugs metabolized by CYP3A4
- Prodrug plus activating enzyme inhibitor
Correct Answer: Beta‑agonist plus beta‑blocker
Q17. What monitoring is most important when combining aminoglycosides with loop diuretics?
- Blood glucose levels
- Hepatic function tests
- Renal function and hearing tests due to nephrotoxicity and ototoxicity risk
- Serum potassium only
Correct Answer: Renal function and hearing tests due to nephrotoxicity and ototoxicity risk
Q18. Which effect results from displacement of a highly protein‑bound drug by another drug?
- Decrease in free drug concentration
- Increase in free (unbound) drug leading to greater pharmacologic effect or toxicity
- Irreversible receptor antagonism
- Enhanced renal excretion without effect on free fraction
Correct Answer: Increase in free (unbound) drug leading to greater pharmacologic effect or toxicity
Q19. Rifampicin is known to be a potent inducer of CYP enzymes. What is the usual clinical consequence when coadministered with drugs metabolized by these enzymes?
- Increased plasma concentration and toxicity
- Reduced plasma concentration and therapeutic failure
- No change in pharmacological activity
- Immediate allergic reactions
Correct Answer: Reduced plasma concentration and therapeutic failure
Q20. Which interaction most commonly causes serotonin syndrome?
- Combining MAO inhibitors with SSRIs or SNRIs
- Combining proton pump inhibitors with H2 blockers
- Combining ACE inhibitors with ARBs
- Combining beta‑blockers with calcium channel blockers
Correct Answer: Combining MAO inhibitors with SSRIs or SNRIs
Q21. Which strategy can minimize clinically significant drug–drug interactions?
- Prescribing multiple drugs with overlapping metabolism without review
- Regular medication reconciliation and reviewing CYP interactions
- Avoiding monitoring even for narrow therapeutic index drugs
- Relying solely on patient self‑report without lab tests
Correct Answer: Regular medication reconciliation and reviewing CYP interactions
Q22. Grapefruit juice primarily affects drug levels by:
- Inducing intestinal CYP3A4 to decrease oral drug levels
- Inhibiting intestinal CYP3A4 and P‑gp, increasing oral bioavailability
- Increasing renal filtration rate
- Altering hepatic blood flow only
Correct Answer: Inhibiting intestinal CYP3A4 and P‑gp, increasing oral bioavailability
Q23. What is the primary concern when combining two QT‑prolonging drugs?
- Enhanced renal clearance
- Increased risk of torsades de pointes and ventricular arrhythmia
- Reduced analgesic efficacy
- Decreased blood pressure variability
Correct Answer: Increased risk of torsades de pointes and ventricular arrhythmia
Q24. Which laboratory parameter is most useful when monitoring interaction risk for a drug with narrow therapeutic index like lithium?
- Serum lithium concentration
- Liver enzymes only
- Hemoglobin A1c
- Platelet count
Correct Answer: Serum lithium concentration
Q25. Which is an example of metabolic activation interaction affecting prodrugs?
- Enzyme inhibition converting prodrug faster to active drug
- Enzyme induction increasing conversion and enhancing effect of a prodrug
- Protein binding displacement of active drug
- Competitive receptor antagonism at the active site
Correct Answer: Enzyme induction increasing conversion and enhancing effect of a prodrug
Q26. Co-administration of NSAIDs with ACE inhibitors may cause which pharmacodynamic interaction?
- Enhanced antihypertensive effect
- Reduced antihypertensive efficacy and increased risk of renal impairment
- Increased metabolic clearance of ACE inhibitor
- Serotonin syndrome
Correct Answer: Reduced antihypertensive efficacy and increased risk of renal impairment
Q27. Why is polypharmacy particularly risky in elderly patients regarding drug interactions?
- Elderly have faster hepatic metabolism preventing interactions
- They have multiple comorbidities, altered pharmacokinetics, and higher susceptibility to adverse interactions
- They are immune to pharmacodynamic interactions
- They have no change in renal function affecting drug clearance
Correct Answer: They have multiple comorbidities, altered pharmacokinetics, and higher susceptibility to adverse interactions
Q28. Which effect would result from coadministering probenecid with penicillin?
- Increased renal tubular secretion of penicillin reducing its levels
- Inhibition of renal excretion of penicillin, prolonging its plasma half‑life
- Induction of hepatic metabolism of penicillin
- Increased biliary excretion of penicillin
Correct Answer: Inhibition of renal excretion of penicillin, prolonging its plasma half‑life
Q29. What clinical action is appropriate when a new drug known to inhibit CYP450 is started in a patient on a narrow therapeutic index drug?
- Ignore potential interaction and continue current dose
- Immediately discontinue the narrow therapeutic index drug without consultation
- Consider dose reduction and monitor plasma levels and clinical response
- Increase the dose of the narrow therapeutic index drug preemptively
Correct Answer: Consider dose reduction and monitor plasma levels and clinical response
Q30. Which interaction exemplifies a transporter‑mediated drug interaction affecting renal excretion?
- Inhibition of organic anion transporters by one drug reducing clearance of another anionic drug
- Induction of CYP450 leading to increased clearance
- Competitive antagonism at the receptor site
- Increased protein binding reducing free fraction
Correct Answer: Inhibition of organic anion transporters by one drug reducing clearance of another anionic drug
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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