Drug Interactions Practice Questions

Q: A patient on warfarin has a high INR. Which drug likely caused this?

Amiodarone. ***Amiodarone*** - Amiodarone is a well-known inhibitor of **CYP2C9**, the primary enzyme responsible for the metabolism of **S-warfarin**, the more potent enantiomer of warfarin. - Inhibition of warfarin metabolism leads to increased warfarin levels, thereby enhancing its anticoagulant effect and causing a **higher INR**. *Phenytoin* - Phenytoin is an **enzyme inducer**, primarily of **CYP2C9** and **CYP3A4**. - Its interaction with warfarin typically leads to **decreased warfarin levels** and a **lower INR**, reducing the anticoagulant effect. *Carbamazepine* - Carbamazepine is a potent **enzyme inducer**, particularly of **CYP3A4** and **CYP2C9**. - Like phenytoin, it generally leads to **increased warfarin metabolism** and a **reduced INR**, thereby decreasing its anticoagulant efficacy. *Rifampicin* - Rifampicin is a strong **inducer of hepatic cytochrome P450 enzymes**, especially **CYP3A4** and **CYP2C9**. - Its co-administration with warfarin significantly **increases warfarin metabolism**, resulting in **lower warfarin concentrations** and a **decreased INR**.

Q: A patient is being treated with warfarin for atrial fibrillation. Which of the following antibiotics has the highest potential to increase warfarin toxicity due to CYP enzyme inhibition?

Ciprofloxacin. ***Ciprofloxacin*** - Among the options listed, **ciprofloxacin** has the most documented potential to increase warfarin effect and bleeding risk. - However, the primary mechanism is **NOT through CYP enzyme inhibition** as stated in the question stem. - Ciprofloxacin's interaction with warfarin occurs through: (1) **disruption of intestinal flora** that produce vitamin K, and (2) possible weak inhibition of **CYP2C9** (the actual enzyme responsible for S-warfarin metabolism). - **Important note:** Ciprofloxacin primarily inhibits **CYP1A2**, which is NOT involved in warfarin metabolism. The question premise has limitations. *Azithromycin* - **Azithromycin** has minimal interaction with the **CYP450 enzyme system**. - Unlike erythromycin and clarithromycin (which inhibit CYP3A4), azithromycin does not significantly inhibit CYP enzymes. - It has a very low potential for clinically significant warfarin interactions. *Doxycycline* - **Doxycycline** has minimal effect on the **CYP450 enzyme system**. - It does not significantly affect **warfarin metabolism** and poses a low risk for drug interactions. - Any reported interactions are likely through disruption of vitamin K-producing gut bacteria rather than CYP inhibition. *Ceftriaxone* - **Ceftriaxone** is primarily eliminated via **renal and biliary excretion**. - It does not interact with the **CYP450 enzyme system**. - It has minimal potential to affect **warfarin metabolism** through CYP pathways. **Clinical Note:** Antibiotics that significantly increase warfarin toxicity through **CYP2C9 inhibition** (the relevant enzyme for warfarin) include **metronidazole**, **sulfamethoxazole-trimethoprim**, and **fluconazole** (antifungal). None of the options in this question are strong CYP2C9 inhibitors.

Q: Which of the following drugs acts as an enzyme inducer, potentially reducing the efficacy of other medications?

Rifampin. ***Rifampin*** - **Rifampin** is a potent **CYP450 enzyme inducer**, particularly of **CYP3A4**, which metabolizes many medications. - This induction accelerates the metabolism of co-administered drugs, leading to **decreased plasma concentrations** and reduced therapeutic efficacy. *Ketoconazole* - **Ketoconazole** is a strong **inhibitor** of **CYP3A4**, meaning it slows down the metabolism of other drugs. - This can lead to **increased plasma concentrations** and potential toxicity of co-administered medications. *Cimetidine* - **Cimetidine** is a **CYP450 enzyme inhibitor**, predominantly affecting **CYP1A2, CYP2C9, CYP2D6,** and **CYP3A4**. - Its inhibitory action can prolong the half-life and increase the toxicity of other drugs, rather than reducing their efficacy through induction. *Chloramphenicol* - **Chloramphenicol** is a **CYP450 enzyme inhibitor**, particularly of **CYP2C19** and **CYP3A4**. - It decreases the metabolic clearance of other drugs, elevating their blood levels and increasing the risk of adverse effects.

Q: Which of the following antimicrobials should not be given to a chronic asthmatic patient managed on theophylline therapy?

Erythromycin. ***Erythromycin*** - **Erythromycin**, a macrolide antibiotic, is a potent inhibitor of the **cytochrome P450 (CYP450) enzyme system**, specifically **CYP1A2**, which is the primary enzyme responsible for theophylline metabolism. - Co-administration of erythromycin can significantly **increase theophylline levels**, leading to toxicity such as **nausea, vomiting, seizures, or cardiac arrhythmias.** - This interaction is clinically significant and erythromycin should be avoided in patients on theophylline therapy. *Amoxicillin* - **Amoxicillin** is a penicillin-class antibiotic that has minimal interaction with theophylline metabolism. - It does not significantly inhibit the **CYP1A2 enzyme** and is generally considered safe to use with theophylline. *Cefotaxime* - **Cefotaxime**, a third-generation cephalosporin, does not significantly affect the metabolism of theophylline. - It does not inhibit **CYP1A2 enzymes** and is safe for use in patients on theophylline therapy. *Cotrimoxazole* - **Cotrimoxazole** (trimethoprim/sulfamethoxazole) may slightly increase theophylline levels by inhibiting some CYP450 isoenzymes, but its effect is generally less pronounced than that of erythromycin. - While caution and monitoring are advised, it is not as strongly contraindicated as erythromycin due to a lower risk of significant toxicity in most cases.

Q: Tadalafil should not be given with:

Vasodilators. ***Vasodilators*** - Tadalafil is a **phosphodiesterase-5 (PDE5) inhibitor** that causes **vasodilation** by increasing cGMP levels, leading to smooth muscle relaxation. - Combining tadalafil with other **vasodilators**, particularly **nitrates**, can lead to a severe and potentially life-threatening drop in **blood pressure (hypotension)**. *Antibiotics* - While some antibiotics, particularly macrolides or azoles, can inhibit **CYP3A4** and increase tadalafil levels, this interaction is typically managed by dose adjustments rather than an absolute contraindication. - The primary concern with administering antibiotics and tadalafil concurrently is **pharmacokinetic interactions**, not a direct pharmacodynamic synergy leading to acute, severe adverse effects. *Vasoconstrictors* - Vasoconstrictors have an effect **opposite** to tadalafil, as they narrow blood vessels. - There is generally no contraindication for co-administration, and in fact, tadalafil's **vasodilatory effects** could potentially **counteract** some of the vasoconstriction, although concurrent use is not typically recommended for erectile dysfunction. *Valproate* - **Valproate** is an **anticonvulsant** and mood stabilizer, and there is no significant or clinically relevant drug interaction established with tadalafil. - It does not share common metabolic pathways or pharmacodynamic effects that would lead to dangerous interactions with tadalafil.

Q: Which of the following medications can increase the nephrotoxicity of Amphotericin B?

All of the options. ***All of the options*** - **Amphotericin B** is known for its nephrotoxic effects, which are exacerbated when co-administered with other nephrotoxic agents. - **Cyclosporin**, **Acyclovir**, and **Vancomycin** are all medications with known nephrotoxic potentials, and their co-administration with Amphotericin B significantly increases the risk of renal damage. *Cyclosporin* - **Cyclosporin** is an immunosuppressant that causes dose-dependent **renal vasoconstriction** and tubular damage, leading to nephrotoxicity. - Concurrent use with Amphotericin B elevates the risk of acute kidney injury due to **additive renal insults**. *Acyclovir* - **Acyclovir**, an antiviral drug, can cause **crystal nephropathy** and direct tubular toxicity, especially with rapid intravenous infusion or in dehydrated patients. - Co-administration with Amphotericin B compounds the likelihood of **renal dysfunction**. *Vancomycin* - **Vancomycin**, an antibiotic, can induce **acute tubular necrosis** and interstitial nephritis, particularly with high trough levels or prolonged therapy. - Its combination with Amphotericin B creates a **synergistic nephrotoxic effect**, increasing the risk of kidney damage.

Q: What is the effect of co-administration of rifampicin and ritonavir in patients suffering from AIDS?

Area Under Curve decreased by 35%. ***Area Under Curve decreased by 35%***- This is the **intended correct answer** for this question, though it should be noted that actual clinical data shows an even more severe interaction.- Rifampicin is a **potent inducer of CYP3A4**, the primary enzyme responsible for ritonavir metabolism.- Co-administration leads to significantly **increased ritonavir clearance** and substantial reduction in its Area Under the Curve (AUC).- **Clinical Note**: In practice, rifampicin reduces ritonavir AUC by approximately **75-85%**, making this combination generally contraindicated or requiring alternative antiretroviral regimens. The 35% figure in this question may reflect a specific study or exam context.*Area Under Curve decreased by 10%*- A 10% decrease would be considered a **clinically minor interaction**, whereas the rifampicin-ritonavir interaction is known to be **severe** due to strong enzyme induction.- This option does not reflect the significant impact of **CYP3A4 induction** by rifampicin on ritonavir's pharmacokinetics.- This magnitude of interaction would not typically require treatment modification.*Area Under Curve increased by 10%*- An increase in AUC would imply that rifampicin is **inhibiting** ritonavir metabolism, which is contrary to its known mechanism as an **enzyme inducer**.- This option is pharmacologically incorrect given rifampicin's role in **CYP450 enzyme induction**.- Rifampicin induces, not inhibits, drug-metabolizing enzymes.*Area Under Curve increased by 20%*- Similar to the 10% increase, a 20% increase in AUC would suggest **enzyme inhibition**, which is not the case with rifampicin.- Rifampicin's primary effect on ritonavir is to **accelerate its metabolism**, leading to reduced, not increased, systemic exposure.- The direction of this interaction is opposite to rifampicin's established pharmacological action.

Q: Which of the following drugs is not an inhibitor of P-glycoprotein?

Phenobarbitone. ***Phenobarbitone*** - **Phenobarbitone** is primarily known as an **inducer** of various metabolic enzymes, including CYP450 enzymes and **P-glycoprotein itself**. - Unlike the other options, it **increases P-glycoprotein expression** (upregulation) rather than inhibiting its function. - This induction leads to enhanced drug efflux and reduced bioavailability of P-gp substrate drugs, which is the opposite effect of P-gp inhibitors [1]. *Quinidine* - **Quinidine** is a well-known and potent **inhibitor of P-glycoprotein**, often used as a reference compound in studies of P-gp modulation. - It can significantly increase the bioavailability and reduce the elimination of P-gp substrate drugs by blocking the efflux pump. *Verapamil* - **Verapamil** is a potent **calcium channel blocker** and also a significant **inhibitor of P-glycoprotein** [2]. - Its P-gp inhibitory activity contributes to clinical drug interactions by increasing the systemic exposure of co-administered P-gp substrate drugs. *Erythromycin* - **Erythromycin**, a macrolide antibiotic, is a known **inhibitor of P-glycoprotein** [3]. - This inhibition can lead to increased concentrations of co-administered P-gp substrates, contributing to potential drug toxicities.

Q: A patient on lithium therapy developed hypertension and was started on a thiazide diuretic. After a few days, he developed coarse tremors and other symptoms suggestive of lithium toxicity. What is the probable mechanism of interaction between thiazide diuretics and lithium?

Thiazide increases the tubular reabsorption of lithium. ***Thiazide increases the tubular reabsorption of lithium*** - Thiazide diuretics cause a decrease in sodium reabsorption in the distal convoluted tubule, leading to increased sodium excretion in urine. - The kidneys, in an attempt to conserve sodium, increase reabsorption in the proximal tubule. Because **lithium** is reabsorbed similarly to sodium in the proximal tubule, this increased reabsorption also affects lithium, leading to a rise in its plasma concentration and toxicity. *Thiazide inhibits the metabolism of lithium* - Lithium is primarily excreted by the kidneys and is not significantly metabolized in the body. - Thiazide diuretics do not affect enzyme systems responsible for drug metabolism. *Thiazides act as an add on the drug to lithium* - This statement is vague and does not explain a mechanism of interaction leading to toxicity. - While both drugs might be prescribed concurrently for different conditions, "add on" does not describe a pharmacological interaction causing altered drug levels. *None of the above* - This option is incorrect because a clear and well-understood mechanism for the interaction between thiazide diuretics and lithium exists.

Question 1

A patient on warfarin has a high INR. Which drug likely caused this?

Accepted Answer

Amiodarone

Answer

Phenytoin

Answer

Carbamazepine

Answer

Rifampicin

Question 2

A patient is being treated with warfarin for atrial fibrillation. Which of the following antibiotics has the highest potential to increase warfarin toxicity due to CYP enzyme inhibition?

Accepted Answer

Ciprofloxacin

Answer

Doxycycline

Answer

Ceftriaxone

Answer

Azithromycin

Question 3

Which of the following drugs acts as an enzyme inducer, potentially reducing the efficacy of other medications?

Accepted Answer

Rifampin

Answer

Ketoconazole

Answer

Cimetidine

Answer

Chloramphenicol

Question 4

Which of the following antimicrobials should not be given to a chronic asthmatic patient managed on theophylline therapy?

Accepted Answer

Erythromycin

Answer

Amoxicillin

Answer

Cefotaxime

Answer

Cotrimoxazole

Question 5

Tadalafil should not be given with:

Accepted Answer

Vasodilators

Answer

Antibiotics

Answer

Vasoconstrictors

Answer

Valproate

Question 6

Which of the following medications can increase the nephrotoxicity of Amphotericin B?

Accepted Answer

All of the options

Answer

Cyclosporin

Answer

Acyclovir

Answer

Vancomycin

Question 7

What is the effect of co-administration of rifampicin and ritonavir in patients suffering from AIDS?

Accepted Answer

Area Under Curve decreased by 35%

Answer

Area Under Curve decreased by 10%

Answer

Area Under Curve increased by 10%

Answer

Area Under Curve increased by 20%

Question 8

Which of the following drug combinations does not exhibit synergistic action?

Accepted Answer

Hydrochlorothiazide and Triamterene (complementary action)

Answer

Levodopa and Carbidopa (enhanced Parkinson's treatment)

Answer

Enalapril and Hydrochlorothiazide (enhanced antihypertensive effect)

Answer

Glibenclamide and Metformin (enhanced glycemic control)

Question 9

Which of the following drugs is not an inhibitor of P-glycoprotein?

Accepted Answer

Phenobarbitone

Answer

Verapamil

Answer

Quinidine

Answer

Erythromycin

Question 10

A patient on lithium therapy developed hypertension and was started on a thiazide diuretic. After a few days, he developed coarse tremors and other symptoms suggestive of lithium toxicity. What is the probable mechanism of interaction between thiazide diuretics and lithium?

Accepted Answer

Thiazide increases the tubular reabsorption of lithium

Answer

Thiazide inhibits the metabolism of lithium

Answer

Thiazides act as an add-on drug to lithium

Answer

None of the above

Drug Interactions — MCQs

Drug Interactions — MCQs

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