Herbal drug–drug interactions with examples MCQs With Answer

Introduction: This quiz-focused blog on herbal drug–drug interactions provides M.Pharm students with a targeted review of clinically important interactions, mechanisms and management strategies. Questions concentrate on pharmacokinetic and pharmacodynamic interactions — including CYP enzymes, P‑glycoprotein and transporter effects, additive toxicities (e.g., bleeding, hepatotoxicity, CNS depression), and representative herbal examples such as St. John’s Wort, grapefruit, ginkgo, garlic, kava, valerian, green tea, milk thistle, licorice and others. Each multiple-choice question includes realistic clinical pairings, concise choices and clear answers to reinforce safe prescribing, monitoring and patient counselling principles for herbal–drug combinations in advanced pharmacy practice.

Q1. Which interaction best explains why St. John’s Wort reduces blood concentrations of cyclosporine?

• Inhibition of renal tubular secretion of cyclosporine
• Induction of CYP3A4 and P‑glycoprotein leading to increased metabolism and efflux
• Competition for plasma protein binding sites
• Inhibition of intestinal uptake transporters (OATP)

Correct Answer: Induction of CYP3A4 and P‑glycoprotein leading to increased metabolism and efflux

Q2. Co‑administration of grapefruit juice with simvastatin most commonly increases risk of rhabdomyolysis by which mechanism?

• Induction of CYP2C9 increasing active metabolites
• Inhibition of intestinal CYP3A4 and OATP leading to higher systemic statin exposure
• Stimulation of renal clearance of statins
• Enhanced hepatic uptake and metabolism via UGT induction

Correct Answer: Inhibition of intestinal CYP3A4 and OATP leading to higher systemic statin exposure

Q3. Which herb is classically associated with increased bleeding risk when taken with warfarin due to antiplatelet effects?

• St. John’s Wort
• Ginkgo biloba
• Milk thistle
• Licorice

Correct Answer: Ginkgo biloba

Q4. A woman on combined oral contraceptives starts St. John’s Wort and experiences breakthrough bleeding. What is the most likely pharmacokinetic mechanism?

• Competition for estrogen receptor binding
• Induction of hepatic CYP3A4 increasing steroid metabolism
• Inhibition of contraceptive absorption by altering gastric pH
• Direct antagonism at progesterone receptor

Correct Answer: Induction of hepatic CYP3A4 increasing steroid metabolism

Q5. Which effect of St. John’s Wort explains reported decreases in digoxin plasma concentrations?

• Inhibition of renal excretion of digoxin
• Induction of intestinal P‑glycoprotein reducing absorption and increasing efflux
• Inhibition of CYP2C9 reducing digoxin metabolism
• Increased protein binding reducing free digoxin levels

Correct Answer: Induction of intestinal P‑glycoprotein reducing absorption and increasing efflux

Q6. Several case reports document increased INR when cranberry juice is consumed by patients on warfarin. Which statement best reflects current clinical advice?

• Cranberry juice consistently decreases warfarin effect and INR
• Cranberry has no interaction with warfarin and requires no monitoring
• Cranberry may increase warfarin effect in some patients; monitor INR closely if used
• Cranberry irreversibly inhibits VKORC1 enhancing warfarin action

Correct Answer: Cranberry may increase warfarin effect in some patients; monitor INR closely if used

Q7. Which herbal supplement is most likely to cause additive CNS depression when combined with benzodiazepines?

• Ginseng (Panax ginseng)
• Valerian (Valeriana officinalis)
• Green tea
• Milk thistle

Correct Answer: Valerian (Valeriana officinalis)

Q8. Which interaction explains why St. John’s Wort can reduce plasma concentrations of HIV protease inhibitors such as indinavir?

• Inhibition of CYP2D6 in the liver
• Induction of intestinal and hepatic CYP3A4 causing increased metabolism
• Chelation of protease inhibitors in the gut lumen
• Inhibition of renal tubular uptake transporters

Correct Answer: Induction of intestinal and hepatic CYP3A4 causing increased metabolism

Q9. A transplant patient on tacrolimus drinks large quantities of grapefruit juice. What is the expected effect on tacrolimus levels?

• Reduced tacrolimus levels due to P‑glycoprotein induction
• No effect; grapefruit only affects statins
• Increased tacrolimus levels due to inhibition of intestinal CYP3A4
• Increased renal clearance of tacrolimus

Correct Answer: Increased tacrolimus levels due to inhibition of intestinal CYP3A4

Q10. Combining St. John’s Wort with an SSRI can lead to which potentially serious adverse effect?

• Hypertensive crisis due to MAO inhibition
• Serotonin syndrome from additive serotonergic activity
• Severe hypoglycaemia
• Acute renal failure due to tubular toxicity

Correct Answer: Serotonin syndrome from additive serotonergic activity

Q11. Excessive intake of green tea can reduce warfarin anticoagulant effect because it contains which component?

• Saponins that inhibit CYP3A4
• High levels of vitamin K that antagonize warfarin
• Alkaloids that displace warfarin from albumin
• Polyunsaturated fatty acids that induce warfarin metabolism

Correct Answer: High levels of vitamin K that antagonize warfarin

Q12. Which herbal product has been most strongly linked to idiosyncratic hepatotoxicity and should be avoided in combination with other hepatotoxic drugs like isoniazid?

• Kava (Piper methysticum)
• Ginkgo biloba
• St. John’s Wort
• Green tea

Correct Answer: Kava (Piper methysticum)

Q13. Milk thistle (silymarin) is often used as a hepatoprotectant. Which of the following is a relevant interaction consideration?

• Silymarin exclusively induces CYP2D6 and has no other effects
• It may modestly inhibit CYP3A4 and P‑glycoprotein, potentially altering metabolism of co‑administered drugs
• It irreversibly binds to warfarin reducing its activity
• It chelates digoxin in the gut reducing absorption

Correct Answer: It may modestly inhibit CYP3A4 and P‑glycoprotein, potentially altering metabolism of co‑administered drugs

Q14. Which herbal supplement can increase bleeding risk when combined with aspirin or clopidogrel due to antiplatelet properties?

• Garlic (Allium sativum)
• Ginseng (Panax ginseng)
• Licorice (Glycyrrhiza glabra)
• Saw palmetto

Correct Answer: Garlic (Allium sativum)

Q15. Curcumin (from turmeric) co‑administered with warfarin or other anticoagulants is a concern because:

• Curcumin is a strong CYP2D6 inducer reducing anticoagulant levels
• Curcumin has antiplatelet and anticoagulant properties that may increase bleeding risk
• Curcumin causes hypokalemia that potentiates anticoagulation
• Curcumin increases vitamin K synthesis in the gut

Correct Answer: Curcumin has antiplatelet and anticoagulant properties that may increase bleeding risk

Q16. Which herbal product has been reported to decrease warfarin anticoagulant effect (lower INR) in some patients by enzyme induction?

• Panax ginseng
• Ginkgo biloba
• Garlic
• Milk thistle

Correct Answer: Panax ginseng

Q17. Goldenseal is known to inhibit which cytochrome enzyme that may increase plasma levels of drugs like certain antidepressants?

• CYP2C9
• CYP2D6
• CYP3A4 exclusively
• UGT1A1

Correct Answer: CYP2D6

Q18. Which juice is known to reduce oral fexofenadine absorption by inhibiting intestinal OATP transporters?

• Orange juice
• Beetroot juice
• Tomato juice
• Grape juice

Correct Answer: Orange juice

Q19. A patient on digoxin and antihypertensive therapy develops hypokalemia and arrhythmia after using a popular herbal remedy. Which herb is the most likely cause?

• Licorice (Glycyrrhiza glabra) causing pseudoaldosteronism and hypokalemia
• Ginkgo biloba causing potassium loss
• St. John’s Wort causing aldosterone blockade
• Valerian causing renal potassium wasting

Correct Answer: Licorice (Glycyrrhiza glabra) causing pseudoaldosteronism and hypokalemia

Q20. A patient on chronic warfarin therapy asks about starting herbal supplements. What is the best immediate clinical approach?

• Advise the patient to stop warfarin and start the herbal supplement
• Recommend any supplement is safe and requires no monitoring
• Review specific herb–warfarin interactions, avoid high‑risk herbs, and increase frequency of INR monitoring if used
• Automatically double the warfarin dose when an herbal product is started

Correct Answer: Review specific herb–warfarin interactions, avoid high‑risk herbs, and increase frequency of INR monitoring if used

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