Drug Interactions Practice Questions

Q: Which of the following agents requires the MOST caution when combined with spironolactone due to increased risk of hyperkalemia:

ACE inhibitors. ***ACE inhibitors*** - Spironolactone is a **potassium-sparing diuretic** that increases potassium levels by blocking aldosterone's effects in the collecting duct [1]. - **ACE inhibitors** also decrease aldosterone production [2], leading to reduced potassium excretion and a significant risk of **severe hyperkalemia** when combined with spironolactone [1, 2].*Beta-blockers* - While beta-blockers can cause a slight increase in plasma potassium by inhibiting cellular potassium uptake, this effect is generally modest and does not pose a major hyperkalemia risk when co-administered with spironolactone. - Their primary interaction concerns blood pressure and heart rate, not direct potassium handling.*Amlodipine* - Amlodipine is a **calcium channel blocker** that primarily causes vasodilation and does not significantly alter potassium balance. - Therefore, it does not substantially increase the risk of hyperkalemia when used concurrently with spironolactone.*Chlorothiazide* - Chlorothiazide is a **thiazide diuretic** that promotes potassium excretion, leading to a risk of hypokalemia. - When combined with spironolactone, a potassium-sparing diuretic, these agents can **partially offset each other's effects** on potassium balance, potentially reducing the risk of hyperkalemia compared to ACE inhibitors.

Q: Which of the following are CYP3A inhibitors?

Both a and c. ***Both a and c (Ritonavir and Verapamil)*** - **Ritonavir** is a **potent CYP3A4 inhibitor**, one of the strongest known, commonly used as a pharmacokinetic booster for other protease inhibitors to increase their bioavailability - **Verapamil** is a **calcium channel blocker** that acts as a **moderate CYP3A4 inhibitor**, leading to clinically significant drug interactions requiring dose adjustments - Both drugs have **clinically relevant and well-established** CYP3A4 inhibitory effects *Ritonavir alone* - While correct that Ritonavir is a potent CYP3A4 inhibitor, this option is incomplete as it excludes Verapamil - Ritonavir's inhibitory effect is so strong that it can increase plasma concentrations of co-administered CYP3A4 substrates by several-fold *Amiodarone* - Amiodarone is primarily a **potent inhibitor of CYP2C9, CYP2D6, and P-glycoprotein** - While it does have **weak to moderate CYP3A4 inhibitory activity**, this effect is **less clinically significant** compared to its effects on other CYP enzymes - In the context of clinically important CYP3A4 inhibitors, Ritonavir and Verapamil are more relevant examples *Verapamil alone* - While correct that Verapamil is a CYP3A4 inhibitor, this option is incomplete as it excludes Ritonavir - Verapamil can increase plasma concentrations of drugs like simvastatin, cyclosporine, and other CYP3A4 substrates

Q: Which of the following reduces the efficacy of oral contraceptives?

Griseofulvin. ***Griseofulvin*** - **Griseofulvin** is an antifungal agent known to induce liver enzymes, specifically the **cytochrome P450 system**. - Enzyme induction accelerates the metabolism and clearance of **oral contraceptives**, leading to lower plasma concentrations and reduced efficacy. *Erythromycin* - **Erythromycin** is a macrolide antibiotic that typically inhibits liver enzymes rather than inducing them. - While it can interfere with the metabolism of some drugs, it usually **increases** rather than decreases the plasma levels of co-administered medications, and is not known to reduce oral contraceptive efficacy. *Disulfiram* - **Disulfiram** is used to treat chronic alcoholism and inhibits aldehyde dehydrogenase. - It does not significantly interact with the metabolism of **oral contraceptives** via the cytochrome P450 system or other mechanisms that would reduce their efficacy. *Cimetidine* - **Cimetidine** is an H2 receptor antagonist that is known to inhibit cytochrome P450 enzymes. - This inhibition would likely **increase** the plasma concentration of drugs metabolized by these enzymes, such as oral contraceptives, rather than reducing their efficacy.

Q: Which of the following medications does not interact with warfarin?

Benzodiazepines. ***Benzodiazepines*** - **Benzodiazepines** are generally considered safe to use with warfarin as they are extensively metabolized in the liver, but they do not typically alter the **cytochrome P450 enzymes** responsible for warfarin metabolism. - They also do not interfere with **vitamin K recycling** or **platelet function**, which are key mechanisms through which other drugs interact with warfarin. *Barbiturate* - **Barbiturates** are **potent inducers of hepatic enzymes**, particularly CYP2C9, which is responsible for metabolizing warfarin. - This enzyme induction leads to **increased warfarin metabolism**, reducing its anticoagulant effect and necessitating higher warfarin doses. *Oral contraceptive* - **Oral contraceptives** can **reduce the anticoagulant effect of warfarin** by inducing clotting factors or inhibiting warfarin metabolism. - This interaction can increase the risk of **thromboembolic events** in patients on warfarin. *Cephalosporins* - Certain **cephalosporins**, especially those with a **methylthiotetrazole (MTT) side chain** (e.g., Cefamandole, Cefoperazone, Moxalactam), can **inhibit vitamin K epoxide reductase**. - This inhibition leads to a **decrease in vitamin K-dependent clotting factors**, thus potentiating the anticoagulant effect of warfarin and increasing bleeding risk.

Q: Which of the following combinations can result in severe toxicity due to inhibition of cytochrome P450 enzymes?

Atorvastatin + Itraconazole. ***Atorvastatin + Itraconazole*** - **Itraconazole** is a potent inhibitor of **CYP3A4**, the primary enzyme responsible for atorvastatin's metabolism. - Co-administration leads to significantly increased **atorvastatin plasma concentrations**, raising the risk of severe side effects like **rhabdomyolysis** and **hepatotoxicity**. *Amiodarone + Atorvastatin* - **Amiodarone** is a moderate **CYP3A4 inhibitor** and can increase atorvastatin levels, but the inhibition is **less potent** than itraconazole. - While this combination does carry a risk and requires dose adjustment, the interaction is **less severe** compared to the potent inhibition seen with itraconazole. - The direct CYP inhibition leading to severe atorvastatin toxicity is less pronounced than with itraconazole. *Carbamazepine + Atorvastatin* - **Carbamazepine** is a potent **CYP3A4 inducer**, meaning it would increase the metabolism of atorvastatin, potentially *decreasing* its efficacy rather than causing toxicity through inhibition. - This interaction would typically lead to subtherapeutic atorvastatin levels, rather than severe toxicity. *Phenytoin + Atorvastatin* - **Phenytoin** is also a potent **CYP3A4 inducer**, similar to carbamazepine. - Concurrent use would likely lead to enhanced metabolism and **reduced efficacy of atorvastatin**, not increased toxicity due to enzyme inhibition.

Q: Which antiretroviral drug should be avoided in a known sputum-positive pulmonary tuberculosis patient who is currently on INH, rifampicin, pyrazinamide, and ethambutol, and has a CD4 count of 100 cells/dL and a viral load of more than 50,000 copies/mL, given that the patient is HIV-positive?

Ritonavir. ***Ritonavir (Correct Answer)*** - **Ritonavir** is the most critical drug to avoid due to severe drug-drug interactions with **rifampicin** - **Rifampicin** is a potent CYP3A4 inducer that dramatically reduces ritonavir plasma concentrations by 75-90%, rendering it completely ineffective - Ritonavir is commonly used as a pharmacokinetic booster for other protease inhibitors, making this interaction particularly significant - **Contraindicated** with rifampicin-based TB regimens *Indinavir* - Also a protease inhibitor metabolized via CYP3A4 - Should also be **avoided with rifampicin** as levels are reduced by approximately 90% - However, indinavir is rarely used in modern ART regimens due to high pill burden, need for dietary restrictions, and significant side effects (nephrolithiasis) - Less commonly used than ritonavir, making ritonavir the better answer *Lamivudine* - Nucleoside reverse transcriptase inhibitor (NRTI) with **renal elimination** - Not metabolized by CYP450 enzymes - **No significant interactions** with rifampicin or other anti-TB drugs - Safe and commonly used in TB/HIV co-infection *Efavirenz* - Non-nucleoside reverse transcriptase inhibitor (NNRTI) that **can be safely co-administered** with rifampicin - Standard dose (600 mg daily) is generally adequate, though some guidelines recommend considering 800 mg in patients >60 kg - **Preferred NNRTI** for TB/HIV co-infection according to WHO guidelines - Well-studied and effective combination with rifampicin-based TB therapy

Q: The antiepileptic drug that does not produce enzyme induction is:

Sodium valproate. **Sodium valproate** - Unlike many other antiepileptic drugs, **sodium valproate is primarily an enzyme inhibitor**, not an enzyme inducer. - It inhibits CYP450 enzymes (particularly CYP2C9) and UGT enzymes, which can **increase levels of co-administered drugs**. - This characteristic makes it important to monitor for potential drug interactions when used with other medications. *Phenobarbitone* - **Phenobarbitone is a potent CYP450 enzyme inducer**, particularly of CYP3A4, CYP2C9, and CYP2C19. - This induction leads to increased metabolism and reduced efficacy of co-administered drugs. *Phenytoin sodium* - **Phenytoin is a well-known CYP450 enzyme inducer**, especially of CYP3A4 and CYP2C9. - Its enzyme-inducing properties necessitate careful monitoring of other medications taken concurrently. *Carbamazepine* - **Carbamazepine is a strong autoinducer** of its own metabolism, as well as an inducer of other CYP450 enzymes. - This enzyme induction leads to a need for dose adjustments over time and significant drug interactions.

Q: Which of the following drugs should not be given with tyramine, as it may result in a dangerous reaction?

Tranylcypromine. ***Tranylcypromine*** - As an **irreversible, non-selective monoamine oxidase inhibitor (MAOI)**, tranylcypromine prevents the breakdown of monoamines, including tyramine. - Ingesting **tyramine-rich foods** while on tranylcypromine can lead to a **hypertensive crisis** due to excessive norepinephrine release. - This is the classic drug-food interaction that requires strict dietary restrictions. *Selegiline* - At **therapeutic doses**, selegiline is a **selective MAO-B inhibitor**, which primarily breaks down dopamine in the brain. - At therapeutic doses, it has **minimal interaction with tyramine** since MAO-A (which metabolizes tyramine in the gut) remains functional. - This selectivity is lost only at higher doses, but at standard dosing, dietary tyramine restrictions are not required. *Dextromethorphan* - This is an **antitussive** (cough suppressant) that acts on the **NMDA receptor** and has weak opioid effects. - It does not interact with tyramine directly, though it can interact with **MAOIs** to cause **serotonin syndrome**. *Meperidine* - This is an **opioid analgesic** that acts on opioid receptors. - While it can interact with **MAOIs** to cause potentially fatal reactions (e.g., **serotonin syndrome** or **hyperpyrexic crisis**), it does not directly interact with dietary tyramine.

Q: In patients taking tadalafil, the most serious drug interaction occurs with:

Nitrates. ***Nitrates*** - The co-administration of **tadalafil** (a PDE5 inhibitor) with **nitrates** can cause a dangerous and potentially fatal drop in **blood pressure**. - Both drug classes lead to increased cGMP levels, resulting in excessive **vasodilation** and profound **hypotension**. *Alpha-Blockers* - Alpha-blockers, while able to cause **hypotension** when taken with tadalafil, generally lead to less severe interactions than nitrates. - The combination requires caution and potentially dose adjustments, but typically does not result in the same life-threatening blood pressure drops as seen with nitrates. *Ketoconazole* - **Ketoconazole** is a strong **CYP3A4 inhibitor**, which can increase the plasma concentration of tadalafil. - This interaction can potentiate tadalafil's effects and increase the risk of side effects, but it doesn't create an immediate, life-threatening hypotensive crisis like nitrates. *Rifampicin* - **Rifampicin** is a potent **CYP3A4 inducer**, which can significantly decrease the plasma concentration of tadalafil. - This interaction primarily leads to a **reduced efficacy** of tadalafil, rather than a dangerous increase in adverse effects or a severe drug-drug interaction.

Q: What is the primary reason that the therapeutic efficacy of antihypertensive drugs is reduced by NSAIDs?

Decrease the synthesis of vascular prostacyclin. ***Decrease the synthesis of vascular prostacyclin*** - NSAIDs **inhibit cyclooxygenase (COX)**, reducing the production of **vasodilatory prostaglandins** such as **prostacyclin (PGI2)** and **PGE2** [1]. This inhibition leads to **vasoconstriction** and **sodium retention**, counteracting the effects of antihypertensive medications [2].*Cause sodium retention* - While NSAIDs do cause **sodium retention**, this is a *consequence* of **prostaglandin inhibition**, not the primary mechanism of reducing antihypertensive efficacy. - **Sodium retention** contributes to increased blood volume and elevated blood pressure, but the foundational issue is the interruption of the **vasodilatory prostaglandin pathway** [3].*Inhibit the action of antihypertensive drugs* - NSAIDs do not directly **inhibit the action** of most antihypertensive drugs at their receptor sites or enzymatic targets. - Instead, they introduce a **counteracting physiological effect** (vasoconstriction and sodium retention) that *reduces the overall clinical efficacy* of the antihypertensive regimen.*Alter the pharmacokinetics of antihypertensive drugs* - NSAIDs generally do not significantly alter the **absorption, distribution, metabolism, or excretion** (ADME) of most antihypertensive drugs to a degree that would profoundly reduce their efficacy. - While some minor pharmacokinetic interactions might occur, the primary mechanism of interaction leading to reduced efficacy is **pharmacodynamic** (physiological counteraction).

Question 1

Which of the following agents requires the MOST caution when combined with spironolactone due to increased risk of hyperkalemia:

Accepted Answer

ACE inhibitors

Answer

Beta-blockers

Answer

Amlodipine

Answer

Chlorothiazide

Question 2

Which of the following are CYP3A inhibitors?

Accepted Answer

Both a and c

Answer

Ritonavir

Answer

Amiodarone

Answer

Verapamil

Question 3

Which of the following reduces the efficacy of oral contraceptives?

Accepted Answer

Griseofulvin

Answer

Disulfiram

Answer

Erythromycin

Answer

Cimetidine

Question 4

Which of the following medications does not interact with warfarin?

Accepted Answer

Benzodiazepines

Answer

Barbiturate

Answer

Oral contraceptive

Answer

Cephalosporins

Question 5

Which of the following combinations can result in severe toxicity due to inhibition of cytochrome P450 enzymes?

Accepted Answer

Atorvastatin + Itraconazole

Answer

Amiodarone + Atorvastatin

Answer

Carbamazepine + Atorvastatin

Answer

Phenytoin + Atorvastatin

Question 6

Which antiretroviral drug should be avoided in a known sputum-positive pulmonary tuberculosis patient who is currently on INH, rifampicin, pyrazinamide, and ethambutol, and has a CD4 count of 100 cells/dL and a viral load of more than 50,000 copies/mL, given that the patient is HIV-positive?

Accepted Answer

Ritonavir

Answer

Indinavir

Answer

Lamivudine

Answer

Efavirenz

Question 7

The antiepileptic drug that does not produce enzyme induction is:

Accepted Answer

Sodium valproate

Answer

Phenobarbitone

Answer

Phenytoin sodium

Answer

Carbamazepine

Question 8

Which of the following drugs should not be given with tyramine, as it may result in a dangerous reaction?

Accepted Answer

Tranylcypromine

Answer

Dextromethorphan

Answer

Meperidine

Answer

Selegiline

Question 9

In patients taking tadalafil, the most serious drug interaction occurs with:

Accepted Answer

Nitrates

Answer

Alpha-Blockers

Answer

Ketoconazole

Answer

Rifampicin

Question 10

What is the primary reason that the therapeutic efficacy of antihypertensive drugs is reduced by NSAIDs?

Accepted Answer

Decrease the synthesis of vascular prostacyclin

Answer

Cause sodium retention

Answer

Inhibit the action of antihypertensive drugs

Answer

Alter the pharmacokinetics of antihypertensive drugs

Drug Interactions — MCQs

Drug Interactions — MCQs

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