Herbal drug–drug interactions MCQs With Answer

Introduction: Understanding herbal drug–drug interactions is essential for M.Pharm students preparing to evaluate patient safety, design studies, and counsel on therapy. Herbal products can alter drug response by pharmacokinetic mechanisms (modifying absorption, metabolism via CYP enzymes or P‑glycoprotein, renal elimination) and pharmacodynamic mechanisms (additive anticoagulant, sedative or serotonergic effects). Many clinically important interactions involve commonly used herbs such as St. John’s wort, grapefruit, ginkgo, garlic, kava, and green tea. This quiz emphasizes mechanistic details, representative clinical examples, and practical implications for drug therapy, helping students integrate biochemical pathways with patient-centered risk management and evidence-based counseling.

Q1. Which mechanism best explains how St. John’s wort reduces plasma concentrations of cyclosporine and some oral contraceptives?

• Competitive inhibition of CYP2D6 in the liver
• Induction of CYP3A4 via pregnane X receptor (PXR) leading to increased metabolism
• Inhibition of intestinal P‑glycoprotein reducing absorption
• Displacement from plasma albumin increasing renal clearance

Correct Answer: Induction of CYP3A4 via pregnane X receptor (PXR) leading to increased metabolism

Q2. Grapefruit juice increases plasma levels of orally administered simvastatin primarily by which mechanism?

• Induction of hepatic CYP3A4, increasing first‑pass metabolism
• Inhibition of intestinal CYP3A4 by furanocoumarins, reducing presystemic metabolism
• Activation of PXR causing enhanced clearance
• Increasing renal excretion via organic anion transporters

Correct Answer: Inhibition of intestinal CYP3A4 by furanocoumarins, reducing presystemic metabolism

Q3. Which herbal product is most commonly associated with increased bleeding risk when taken with warfarin due to antiplatelet or anticoagulant effects?

• Ginkgo biloba
• St. John’s wort
• Milk thistle (Silybum marianum)
• Echinacea

Correct Answer: Ginkgo biloba

Q4. Which herb is known to reduce warfarin anticoagulant effect by inducing drug‑metabolizing enzymes and thereby lowering INR?

• St. John’s wort
• Garlic (Allium sativum)
• Green tea (Camellia sinensis)
• Ginkgo biloba

Correct Answer: St. John’s wort

Q5. A patient on digoxin begins taking an herbal remedy and later has reduced digoxin levels and loss of efficacy. Which interaction mechanism is the most plausible?

• Herbal inhibition of renal tubular secretion of digoxin
• Induction of intestinal P‑glycoprotein (P‑gp) increasing efflux and lowering absorption
• Displacement of digoxin from tissue binding sites
• Competitive inhibition of digoxin glucuronidation

Correct Answer: Induction of intestinal P‑glycoprotein (P‑gp) increasing efflux and lowering absorption

Q6. Combining St. John’s wort with an SSRI carries a documented risk of which pharmacodynamic adverse effect?

• Hypoglycemia
• Serotonin syndrome
• Prolonged QT due to potassium loss
• Agranulocytosis

Correct Answer: Serotonin syndrome

Q7. Kava (Piper methysticum) can interact dangerously with benzodiazepines by which mechanism?

• Inducing CYP3A4 and reducing benzodiazepine levels
• Additive CNS‑depressant (sedative) pharmacodynamic effect
• Increasing renal clearance of benzodiazepines
• Antagonizing benzodiazepine receptor function

Correct Answer: Additive CNS‑depressant (sedative) pharmacodynamic effect

Q8. Goldenseal (Hydrastis canadensis) is known to affect drug levels by inhibiting which enzymes?

• CYP1A2 and CYP2E1
• CYP2D6 and CYP3A4
• UDP‑glucuronosyltransferase only
• CYP2C19 exclusively

Correct Answer: CYP2D6 and CYP3A4

Q9. Which herb has been repeatedly implicated in reducing levels of immunosuppressants such as cyclosporine and tacrolimus, risking transplant rejection?

• Black cohosh
• St. John’s wort
• Milk thistle
• Valerian

Correct Answer: St. John’s wort

Q10. Milk thistle (silymarin) is often used for hepatoprotection. Which statement about its drug interaction potential is most accurate?

• It is a strong inducer of CYP3A4 and lowers drug levels markedly
• It is a potent inhibitor of CYP2D6 causing life‑threatening interactions
• It may weakly inhibit CYP3A4 and CYP2C9 in vitro, with variable clinical relevance
• It irreversibly inhibits P‑glycoprotein transporters

Correct Answer: It may weakly inhibit CYP3A4 and CYP2C9 in vitro, with variable clinical relevance

Q11. Which herbal supplement is well documented to increase bleeding risk when combined with anticoagulants or antiplatelet agents due to platelet inhibition and fibrinolytic effects?

• Garlic (Allium sativum)
• St. John’s wort
• Black cohosh
• Milk thistle

Correct Answer: Garlic (Allium sativum)

Q12. A patient on stable warfarin therapy starts drinking large amounts of green tea daily and their INR decreases. What is the most likely explanation?

• Green tea contains vitamin K, which antagonizes warfarin
• Green tea induces CYP3A4 dramatically increasing warfarin clearance
• Green tea inhibits warfarin absorption from the gut
• Green tea increases renal elimination of warfarin metabolites

Correct Answer: Green tea contains vitamin K, which antagonizes warfarin

Q13. Which herbal product has been shown to reduce the efficacy of oral contraceptives through enzyme induction and should be considered when counseling patients about contraceptive failure?

• Ginkgo biloba
• St. John’s wort
• Valerian root
• Turmeric (curcumin)

Correct Answer: St. John’s wort

Q14. Licorice (Glycyrrhiza glabra) can increase the risk of digoxin toxicity by which intermediary physiological change?

• Inducing CYP3A4 to increase digoxin clearance
• Causing pseudoaldosteronism with hypokalemia, enhancing cardiac glycoside toxicity
• Directly inhibiting digoxin renal secretion
• Increasing plasma albumin and lowering free digoxin

Correct Answer: Causing pseudoaldosteronism with hypokalemia, enhancing cardiac glycoside toxicity

Q15. The grapefruit constituent most responsible for CYP3A4 inhibition and clinical interactions is which type of compound?

• Flavonoid glycosides (e.g., hesperidin)
• Furanocoumarins (e.g., bergamottin)
• Alkaloids (e.g., bergapten)
• Saponins

Correct Answer: Furanocoumarins (e.g., bergamottin)

Q16. Kava has been associated with liver injury. Co‑administration with which of the following would most likely increase hepatotoxic risk?

• Proton pump inhibitors (no known hepatotoxicity)
• Acetaminophen and chronic alcohol use
• Topical antibiotics applied to the skin
• Oral contraceptives alone

Correct Answer: Acetaminophen and chronic alcohol use

Q17. Curcumin (turmeric) has in vitro activity on several CYP enzymes. Which effect could increase a patient’s warfarin sensitivity?

• Curcumin induction of CYP2C9 increasing warfarin clearance
• Curcumin inhibition of CYP2C9 decreasing warfarin metabolism and increasing INR
• Curcumin increasing renal excretion of warfarin
• Curcumin displacing warfarin from enterohepatic circulation only

Correct Answer: Curcumin inhibition of CYP2C9 decreasing warfarin metabolism and increasing INR

Q18. A patient on HIV protease inhibitors develops loss of viral control after starting an herbal supplement. Which herb is most likely responsible by inducing CYP3A4 and lowering antiretroviral levels?

• St. John’s wort
• Milk thistle
• Ginkgo biloba
• Valerian

Correct Answer: St. John’s wort

Q19. Which herbal supplement is commonly recommended to be discontinued at least 1–2 weeks prior to elective surgery because of increased intraoperative bleeding risk?

• Ginkgo biloba
• St. John’s wort
• Milk thistle
• American ginseng

Correct Answer: Ginkgo biloba

Q20. Echinacea is sometimes reported to interact with conventional drugs. Which general pattern summarizes its variable interaction profile?

• Consistent and strong inhibition of P‑glycoprotein causing marked drug accumulation
• Variable, time‑dependent effects on CYP enzymes (transient induction or inhibition), leading to unpredictable interactions
• Irreversible inhibition of UDP‑glucuronosyltransferase family enzymes
• Absolute safety with no reported interactions

Correct Answer: Variable, time‑dependent effects on CYP enzymes (transient induction or inhibition), leading to unpredictable interactions

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