Drug Interactions Practice Questions

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).

Q: A 20-year-old nulliparous woman is taking oral contraceptive pills. Which anti-tuberculous drug decreases the effect of OCP?

Rifampicin. ***Rifampicin*** - **Rifampicin** is a potent **CYP450 enzyme inducer**, accelerating the metabolism of oral contraceptive pills [1]. - This increased metabolism leads to lower systemic levels of contraceptive hormones, thereby reducing the OCP's effectiveness and increasing the risk of **unintended pregnancy**. *INH (Isoniazid)* - Isoniazid is primarily metabolized by N-acetyltransferase and cytochrome P450, but it does **not significantly induce** the CYP450 enzymes responsible for OCP metabolism. - It has a much lower potential for drug-drug interactions with oral contraceptives compared to rifampicin. *Pyrazinamide* - Pyrazinamide's metabolism does **not involve significant CYP450 induction** or inhibition that would affect the efficacy of oral contraceptives. - Its mechanism of action and metabolic pathways do not typically interfere with hormonal contraception. *Ethambutol* - Ethambutol is largely excreted unchanged and has **minimal to no interaction with the cytochrome P450 system**. - It does **not affect the metabolism** or effectiveness of oral contraceptive pills.

Q: Which antibiotic should not be taken with milk?

Tetracycline. ***Tetracycline*** - **Tetracycline** forms **insoluble chelates** with divalent and trivalent cations such as calcium (in milk), iron, magnesium, and aluminum. - This chelation significantly **reduces the absorption** of tetracycline from the gastrointestinal tract, diminishing its effectiveness. *Chloramphenicol* - **Chloramphenicol** absorption is generally not significantly affected by milk or food intake. - It is known for its potential to cause **bone marrow suppression** and **aplastic anemia**, unrelated to milk interactions. *Erythromycin* - Some forms of **erythromycin** (e.g., erythromycin stearate) are better absorbed on an empty stomach, but milk itself does not typically form insoluble complexes that drastically reduce absorption like with tetracyclines. - It can cause gastrointestinal side effects like **nausea** and **abdominal cramps**. *Sulfonamide* - **Sulfonamides** are generally well-absorbed and their absorption is not significantly impacted by milk. - They are known for potential side effects such as **allergic reactions** and **crystalluria**.

Q: Lenalidomide co-administration with which other drug increases the risk of thrombosis?

Glucocorticoids. ***Glucocorticoids*** - Co-administration of **lenalidomide** with **dexamethasone** (a glucocorticoid) significantly increases the risk of **venous thromboembolism (VTE)** in patients with **multiple myeloma**. - This combination therapy, though effective, necessitates **thromboprophylaxis** to mitigate the heightened risk of blood clots. *Alcohol* - While alcohol can affect drug metabolism and contribute to bleeding risk, it is not specifically known to increase the **thrombotic risk** when co-administered with lenalidomide. - Its primary interactions with lenalidomide are not related to clotting. *Barbiturates* - **Barbiturates** are strong **CYP450 enzyme inducers**, which can alter the metabolism of many drugs by decreasing their plasma concentrations. - They are not directly associated with an increased **thrombosis risk** when combined with lenalidomide. *Antibiotics* - Some antibiotics can interact with coagulation pathways or affect vitamin K levels, but there is no specific evidence linking **antibiotic co-administration** with an increased **thrombotic risk** when taken with lenalidomide. - Any potential interactions are generally minor and not related to thrombosis risk.

Q: Which drug should not be given with ketoconazole?

Indinavir. ***Correct: Indinavir*** - **Indinavir** is a **protease inhibitor (antiretroviral)** that is primarily metabolized by **CYP3A4** - **Ketoconazole** is a **potent CYP3A4 inhibitor** that significantly increases indinavir plasma concentrations - Co-administration leads to **increased risk of indinavir toxicity** including nephrolithiasis, hyperbilirubinemia, and hepatotoxicity - **Dose reduction of indinavir is required** if concurrent use is necessary (typically reduce to 600 mg q8h from 800 mg q8h) *Incorrect: Macrolide* - Many **macrolides** (erythromycin, clarithromycin) are CYP3A4 substrates and can interact with ketoconazole - While caution is advised due to **QT prolongation risk**, this interaction is less severe than with indinavir - Not an absolute contraindication but requires monitoring *Incorrect: Aminoglycoside* - **Aminoglycosides** (gentamicin, amikacin, tobramycin) are **NOT metabolized by CYP450 enzymes** - They are **hydrophilic** and eliminated **unchanged by renal excretion** - **No clinically significant interaction** with ketoconazole - Can be safely co-administered without dose adjustment *Key Learning Point* - Ketoconazole inhibits CYP3A4, affecting metabolism of many drugs including **protease inhibitors, calcium channel blockers, statins, and some macrolides** - Always check for CYP3A4 substrate drugs when prescribing azole antifungals

Q: Which of the following drugs has the least significant drug interaction with digoxin?

Amlodipine. ***Amlodipine*** - **Amlodipine** generally has a **minimal impact** on digoxin levels and rarely causes clinically significant interactions. - It works through a different mechanism (calcium channel blockade) and does not typically affect digoxin's absorption, distribution, metabolism, or excretion. *Cholestyramine* - **Cholestyramine** is an **ion-exchange resin** that can bind to digoxin in the gastrointestinal tract, significantly **reducing its absorption**. - This interaction can lead to **subtherapeutic digoxin levels** and reduced therapeutic efficacy. *Thiazide diuretics* - **Thiazide diuretics** can cause **hypokalemia**, which significantly **potentiates digoxin toxicity**. - Low potassium levels allow more digoxin to bind to the Na+/K+-ATPase pump, increasing its inotropic and arrhythmogenic effects. *Quinidine* - **Quinidine** can **increase serum digoxin concentrations** by inhibiting its renal and non-renal clearance and displacing it from tissue binding sites. - This can lead to **digoxin toxicity**, necessitating a reduction in digoxin dosage when co-administered.

Q: Concomitant treatment with which of the following can cause oral contraceptive failure?

Rifampicin. ***Rifampicin*** - **Rifampicin** is a potent inducer of **hepatic microsomal enzymes** (cytochrome P450 enzymes), particularly CYP3A4. - This enzyme induction leads to increased metabolism and faster clearance of the estrogen and progestin components of **oral contraceptives**, reducing their effectiveness and risking unintended pregnancy. *Enalapril* - **Enalapril** is an ACE inhibitor used to treat hypertension and heart failure; it does not significantly interact with **oral contraceptive metabolism**. - Its primary mechanism of action involves the **renin-angiotensin-aldosterone system**, which is distinct from the hepatic metabolism of sex hormones. *Ibuprofen* - **Ibuprofen** is a nonsteroidal anti-inflammatory drug (**NSAID**) used for pain and inflammation; it does not affect the metabolism of **oral contraceptives**. - Its mechanism involves inhibiting **prostaglandin synthesis**, and it is not an inducer of hepatic enzymes. *Metformin* - **Metformin** is an oral hypoglycemic agent used for type 2 diabetes; it does not have known significant interactions that would cause **oral contraceptive failure**. - Its primary action is to reduce **hepatic glucose production** and improve insulin sensitivity, with no impact on sex hormone metabolism.

Question 1

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

Accepted Answer

Nitrates

Answer

Alpha-Blockers

Answer

Ketoconazole

Answer

Rifampicin

Question 2

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

Question 3

Oral contraceptive failure occurs in a patient on rifampicin due to which of the following reasons?

Accepted Answer

Rifampicin induces the metabolism of contraceptive hormones.

Answer

Rifampicin stimulates gonadotropin release from the pituitary.

Answer

Rifampicin decreases the secretion of progestin.

Answer

Rifampicin antagonizes the action of oral contraceptives.

Question 4

A 20-year-old nulliparous woman is taking oral contraceptive pills. Which anti-tuberculous drug decreases the effect of OCP?

Accepted Answer

Rifampicin

Answer

INH

Answer

Pyrazinamide

Answer

Ethambutol

Question 5

Which antibiotic should not be taken with milk?

Accepted Answer

Tetracycline

Answer

Chloramphenicol

Answer

Erythromycin

Answer

Sulfonamide

Question 6

Lenalidomide co-administration with which other drug increases the risk of thrombosis?

Accepted Answer

Glucocorticoids

Answer

Alcohol

Answer

Barbiturates

Answer

Antibiotics

Question 7

Which of the following statements is true regarding a fixed-dose combination of drugs?

Accepted Answer

The adverse effect of one drug may be mitigated by the other drug.

Answer

The dose of one drug can be adjusted independently.

Answer

Adverse effects can be attributed solely to one drug.

Answer

Combining two drugs with different pharmacokinetics is straightforward.

Question 8

Which drug should not be given with ketoconazole?

Accepted Answer

Indinavir

Answer

Macrolide

Answer

All of the options

Answer

Aminoglycoside

Question 9

Which of the following drugs has the least significant drug interaction with digoxin?

Accepted Answer

Amlodipine

Answer

Cholestyramine

Answer

Thiazide diuretics

Answer

Quinidine

Question 10

Concomitant treatment with which of the following can cause oral contraceptive failure?

Accepted Answer

Rifampicin

Answer

Enalapril

Answer

Ibuprofen

Answer

Metformin

Drug Interactions — MCQs

Drug Interactions — MCQs

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