Drug Interactions Practice Questions

Q: Tadalafil should not be given with which of the following drugs?

Nitroglycerin. ***Nitroglycerin*** * Tadalafil is a **phosphodiesterase-5 (PDE5) inhibitor** used for erectile dysfunction and pulmonary hypertension. * **All nitrates** including nitroglycerin are **absolutely contraindicated** with PDE5 inhibitors due to synergistic vasodilation causing **severe, potentially fatal hypotension**. * The mechanism involves both drugs increasing cGMP levels through different pathways, leading to profound blood pressure drop. *Hydralazine* * Hydralazine is a **direct arterial vasodilator** acting through different mechanisms (increasing cGMP via nitric oxide). * While caution is advised, it is **not an absolute contraindication** with tadalafil. Blood pressure should be monitored but concurrent use is possible. *Amlodipine* * Amlodipine is a **calcium channel blocker** causing vasodilation by blocking calcium influx into vascular smooth muscle. * **Not contraindicated** with PDE5 inhibitors. May cause additive hypotensive effects requiring dose adjustment, but can be used together safely with monitoring. *Losartan* * Losartan is an **angiotensin II receptor blocker (ARB)** used for hypertension. * **No contraindication** with tadalafil. Both can be safely used together with routine blood pressure monitoring.

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 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 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.

Question 1

Tadalafil should not be given with which of the following drugs?

Accepted Answer

Nitroglycerin

Answer

Amlodipine

Answer

Hydralazine

Answer

Losartan

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 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 4

Which of the following reduces the efficacy of oral contraceptives?

Accepted Answer

Griseofulvin

Answer

Disulfiram

Answer

Erythromycin

Answer

Cimetidine

Question 5

Which of the following medications does not interact with warfarin?

Accepted Answer

Benzodiazepines

Answer

Barbiturate

Answer

Oral contraceptive

Answer

Cephalosporins

Question 6

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 7

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 8

The antiepileptic drug that does not produce enzyme induction is:

Accepted Answer

Sodium valproate

Answer

Phenobarbitone

Answer

Phenytoin sodium

Answer

Carbamazepine

Question 9

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 10

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

Accepted Answer

Nitrates

Answer

Alpha-Blockers

Answer

Ketoconazole

Answer

Rifampicin

Drug Interactions — MCQs

Drug Interactions — MCQs

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