Drug Interactions Practice Questions

Q: Which anti-asthma drug is avoided with erythromycin?

Theophylline. ***Theophylline*** - **Erythromycin** inhibits the **cytochrome P450 (CYP) enzymes** responsible for theophylline metabolism, leading to increased theophylline levels and potential toxicity. - Elevated theophylline can cause adverse effects such as **nausea**, **vomiting**, **arrhythmias**, and **seizures**. *Ipratropium* - **Ipratropium** is largely eliminated unchanged in urine, with minimal hepatic metabolism. - It does not interact significantly with **erythromycin** as its metabolism is not dependent on the CYP enzyme system. *Salbutamol* - **Salbutamol** is primarily metabolized by **sulfotransferase enzymes** in the liver and gut, not primarily by CYP enzymes. - Therefore, **erythromycin** has a negligible impact on salbutamol's metabolism and plasma levels. *Terbutaline* - **Terbutaline** is mainly metabolized by **conjugation reactions** (glucuronidation and sulfation) and excreted renally. - It also has limited interaction potential with **erythromycin** due to its distinct metabolic pathways.

Q: A lady has taken medication for amoebiasis infection. She drank alcohol on the same day. She has nausea, vomiting, and dizziness. Which anti-amoebic drug could have led to interaction with alcohol to produce these symptoms?

Metronidazole. ***Metronidazole*** - **Metronidazole** is well-known for causing a **disulfiram-like reaction** when consumed with alcohol. - This reaction leads to a rapid accumulation of **acetaldehyde**, manifesting as nausea, vomiting, flushing, headache, and dizziness. *Nitazoxanide* - **Nitazoxanide** is an oral anti-infective agent used for protozoal infections, but it is **not associated with a disulfiram-like reaction** with alcohol. - Its mechanism of action involves interfering with the **pyridoxine ferredoxin oxidoreductase enzyme**, which is distinct from alcohol metabolism. *Paromomycin* - **Paromomycin** is an aminoglycoside antibiotic primarily used for luminal amoebiasis and is **poorly absorbed from the GI tract**. - It does **not typically interact with alcohol** or cause disulfiram-like reactions. *Diloxanide* - **Diloxanide furoate** is a luminal amoebicide primarily used for asymptomatic cyst passers. - It is **not known to cause a disulfiram-like reaction** or significant interactions with alcohol.

Q: Spironolactone should be used with extreme caution or avoided with which of the following drugs?

ACE inhibitor (e.g., Enalapril). ***ACE inhibitor (e.g., Enalapril)*** - The combination of **spironolactone**, a **potassium-sparing diuretic**, and an **ACE inhibitor** significantly increases the risk of **hyperkalemia** due to their synergistic effects on potassium retention. - Both drug classes reduce **aldosterone activity** and lead to decreased potassium excretion, potentially causing life-threatening cardiac arrhythmias. *Beta-blocker (e.g., Atenolol)* - While beta-blockers can affect potassium levels, the risk of severe **hyperkalemia** when combined with **spironolactone** is usually manageable and does not represent a direct contraindication. - This combination is often used in managing conditions like **heart failure**, with careful monitoring. *Thiazide diuretic (e.g., Chlorthiazide)* - **Thiazide diuretics** tend to cause **hypokalemia**, and spironolactone can be used in combination to **counteract** this potassium loss. - This combination is often beneficial, as spironolactone helps to conserve potassium while the thiazide diuretic promotes diuresis, reducing the overall risk of electrolyte imbalance. *Calcium channel blocker (e.g., Verapamil)* - Calcium channel blockers primarily affect **calcium influx** into cells and have no direct significant interaction with spironolactone regarding potassium levels. - There is no contraindication for combining these two drug classes, and they are frequently used together safely for various cardiovascular conditions.

Q: Severe hyperkalemia is seen in combination with:

Losartan and amiloride. ***Losartan and amiloride*** - **Losartan** is an **angiotensin receptor blocker (ARB)**, often leading to **hyperkalemia** by inhibiting aldosterone secretion. - **Amiloride** is a **potassium-sparing diuretic**, directly interfering with sodium reabsorption and potassium excretion in the **collecting duct**. The combination of Na⁺-channel inhibitors like amiloride with angiotensin-converting enzyme inhibitors (or ARBs) carries a risk of life-threatening hyperkalemia [1].*Amiloride and Furosemide* - **Furosemide** is a **loop diuretic** that causes **potassium wasting**, thus counteracting the potassium-sparing effect of amiloride [2]. - Combining these two drugs typically results in a **neutral or mildly hypokalemic effect**, not severe hyperkalemia.*Lisinopril and furosemide* - **Lisinopril** is an **ACE inhibitor** that causes **hyperkalemia** by reducing aldosterone levels. - However, **furosemide** induces **hypokalemia**, significantly mitigating the hyperkalemic potential of lisinopril [2].*Propranolol and verapamil* - **Propranolol** (a beta-blocker) and **verapamil** (a calcium channel blocker) primarily affect heart rate and contractility, and blood pressure. - Neither of these drugs is directly associated with significant alterations in **potassium levels** to cause severe hyperkalemia.

Q: Which antibiotic potentiates the effect of neuromuscular blockers?

Aminoglycoside. ***Aminoglycoside*** - **Aminoglycosides** inhibit presynaptic release of **acetylcholine** and suppress postsynaptic sensitivity to acetylcholine, thereby enhancing the effects of **neuromuscular blockers (NMBs)**. - This potentiation can lead to prolonged **neuromuscular blockade** and **respiratory depression**, especially in patients with impaired renal function or pre-existing neuromuscular disease. *Erythromycin* - **Erythromycin**, a **macrolide antibiotic**, is primarily known for inhibiting the **cytochrome P450 3A4 (CYP3A4)** enzyme, which can alter the metabolism of many drugs. - It does not directly impact **acetylcholine** release or receptor sensitivity to the extent that it significantly potentiates **neuromuscular blockers**. *Nitrofurantoin* - **Nitrofurantoin** is an **antibiotic** used predominantly for **urinary tract infections** and acts by damaging bacterial DNA and ribosomal proteins. - There is no known significant interaction or potentiation effect of **nitrofurantoin** on **neuromuscular blockers**. *Co-trimoxazole* - **Co-trimoxazole (trimethoprim-sulfamethoxazole)** is a combination **antibiotic** that interferes with bacterial **folic acid synthesis**. - It does not interfere with **neuromuscular transmission** or potentiate **neuromuscular blockers**, unlike aminoglycosides.

Q: A lady gets pregnant even though she was on contraceptive pills. She is suspected to have consumed

Rifampicin. ***Rifampicin*** - **Rifampicin** is a potent inducer of **hepatic microsomal enzymes** (cytochrome P450 enzymes), particularly CYP3A4. - This enzyme induction leads to increased metabolism and thus decreased effectiveness of **oral contraceptive pills**, raising the risk of unintended pregnancy. *Ciprofloxacin* - **Ciprofloxacin** is a **quinolone antibiotic** that primarily works by inhibiting bacterial DNA gyrase and topoisomerase IV. - It does not significantly induce hepatic enzymes or interfere with the efficacy of **oral contraceptive pills**. *Streptomycin* - **Streptomycin** is an **aminoglycoside antibiotic** that inhibits bacterial protein synthesis. - It is not known to have a significant drug interaction with **oral contraceptive pills** that would lead to contraceptive failure. *None of these* - This option is incorrect because **Rifampicin** is well-documented to reduce the effectiveness of **oral contraceptive pills**.

Q: A patient is on warfarin therapy. All of the following drugs increase the risk of bleeding with warfarin except?

Carbamazepine. ***Carbamazepine*** - Carbamazepine **induces cytochrome P450 enzymes**, specifically **CYP3A4** and **CYP2C9**, which are responsible for warfarin metabolism. - This induction leads to a **faster metabolism of warfarin**, thus **decreasing its anticoagulant effect** and thereby reducing the risk of bleeding. *Isoniazid* - Isoniazid is an **inhibitor of cytochrome P450 enzymes**, primarily **CYP2C9**, which metabolizes the more potent S-warfarin isomer. - This inhibition **decreases warfarin metabolism**, leading to **increased anticoagulant effect** and higher risk of bleeding. *Amiodarone* - Amiodarone is a potent **inhibitor of cytochrome P450 enzymes**, significantly **CYP2C9** and **CYP3A4**. - It leads to a **reduced metabolism of warfarin**, causing **elevated INR** and an increased risk of bleeding. *Cimetidine* - Cimetidine is a known **inhibitor of various cytochrome P450 enzymes**, particularly **CYP1A2**, **CYP2C9**, and **CYP3A4**. - Its inhibitory action on warfarin metabolism results in **higher warfarin levels** and an **increased risk of bleeding**.

Q: A 75-year-old male on warfarin is prescribed an antibiotic for pneumonia. Which antibiotic requires INR monitoring due to increased bleeding risk?

Ciprofloxacin. ***Ciprofloxacin*** - **Ciprofloxacin** and other fluoroquinolones can interact with **warfarin**, though the mechanism is **not well-established** and likely multifactorial (may involve gut flora disruption, protein binding displacement, or metabolic effects). - This interaction can lead to **increased INR** and bleeding risk, requiring close monitoring. - Among fluoroquinolones, the interaction is **less predictable** compared to some other antibiotics. *Amoxicillin* - **Amoxicillin** and other beta-lactam antibiotics can interact with warfarin through **gut flora disruption**, reducing vitamin K synthesis. - This can lead to increased INR, though the effect is generally **mild to moderate**. - Routine INR monitoring is typically sufficient without intensive monitoring. *Clindamycin* - **Clindamycin** has **minimal documented interaction** with warfarin. - It does not significantly affect warfarin metabolism or vitamin K synthesis. - Generally considered a **safer option** for patients on warfarin therapy. *Azithromycin* - **Azithromycin** is well-documented to **significantly increase INR** and bleeding risk in patients on warfarin. - The mechanism may involve **CYP3A4 effects** and other pharmacokinetic interactions. - **FDA warnings exist** regarding this interaction, and close INR monitoring is essential. - Note: While this option is also clinically significant, the question focuses on identifying ONE antibiotic requiring monitoring, with ciprofloxacin being the presented answer in this educational context.

Q: A patient on rifampin for TB starts birth control pills. What adjustment is needed?

Switch to non-hormonal contraception. ***Switch to non-hormonal contraception*** - **Rifampin** is a potent **CYP450 enzyme inducer**, which significantly increases the metabolism of **estrogen and progestin** in birth control pills. - This interaction can lead to **reduced effectiveness** of oral contraceptives, increasing the risk of **unintended pregnancy**, making non-hormonal options more reliable. *Add a second hormonal contraceptive* - Adding another hormonal contraceptive in conjunction with the birth control pills would not circumvent the problem of **rifampin's enzyme-inducing effect** on hormonal metabolism. - This approach would still leave the patient at an **increased risk of contraceptive failure** due to rapid drug metabolism. *Increase birth control dose* - While increasing the dose might partially compensate for increased metabolism, the extent of **enzyme induction by rifampin** can be unpredictable and variable, making it difficult to find an effective and safe dose. - A higher dose of hormones can also lead to an **increased risk of side effects** such as **venous thromboembolism**. *No change needed* - This is incorrect because **rifampin significantly reduces the effectiveness of hormonal contraception** through enzyme induction. - Failing to make an adjustment would expose the patient to a **high risk of unintended pregnancy**.

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**.

Question 1

Which anti-asthma drug is avoided with erythromycin?

Accepted Answer

Theophylline

Answer

Ipratropium

Answer

Salbutamol

Answer

Terbutaline

Question 2

A lady has taken medication for amoebiasis infection. She drank alcohol on the same day. She has nausea, vomiting, and dizziness. Which anti-amoebic drug could have led to interaction with alcohol to produce these symptoms?

Accepted Answer

Metronidazole

Answer

Nitazoxanide

Answer

Paromomycin

Answer

Diloxanide

Question 3

Spironolactone should be used with extreme caution or avoided with which of the following drugs?

Accepted Answer

ACE inhibitor (e.g., Enalapril)

Answer

Beta-blocker (e.g., Atenolol)

Answer

Thiazide diuretic (e.g., Chlorthiazide)

Answer

Calcium channel blocker (e.g., Verapamil)

Question 4

Severe hyperkalemia is seen in combination with:

Accepted Answer

Losartan and amiloride

Answer

Amiloride and Furosemide

Answer

Lisinopril and Furosemide

Answer

Propranolol and verapamil

Question 5

Which antibiotic potentiates the effect of neuromuscular blockers?

Accepted Answer

Aminoglycoside

Answer

Erythromycin

Answer

Nitrofurantoin

Answer

Co-trimoxazole

Question 6

A lady gets pregnant even though she was on contraceptive pills. She is suspected to have consumed

Accepted Answer

Rifampicin

Answer

Ciprofloxacin

Answer

Streptomycin

Answer

None of these

Question 7

A patient is on warfarin therapy. All of the following drugs increase the risk of bleeding with warfarin except?

Accepted Answer

Carbamazepine

Answer

Isoniazid

Answer

Amiodarone

Answer

Cimetidine

Question 8

A 75-year-old male on warfarin is prescribed an antibiotic for pneumonia. Which antibiotic requires INR monitoring due to increased bleeding risk?

Accepted Answer

Ciprofloxacin

Answer

Amoxicillin

Answer

Clindamycin

Answer

Azithromycin

Question 9

A patient on rifampin for TB starts birth control pills. What adjustment is needed?

Accepted Answer

Switch to non-hormonal contraception

Answer

Add a second hormonal contraceptive

Answer

Increase birth control dose

Answer

No change needed

Question 10

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

Accepted Answer

Amiodarone

Answer

Phenytoin

Answer

Carbamazepine

Answer

Rifampicin

Drug Interactions — MCQs

Drug Interactions — MCQs

On this page

Practice by Chapter

Want unlimited practice?