Drug Interactions Practice Questions

Q: A patient who was diagnosed with epilepsy was put on retigabine TDS. Now phenytoin is being added. Which of the following changes should be made to retigabine?

Increase the dose. ***Increase the dose*** - **Phenytoin** is a potent **enzyme inducer** that induces hepatic enzymes including **UGT (glucuronidation) enzymes**. - **Retigabine** is primarily metabolized by **N-acetylation and glucuronidation** (not significantly by CYP450 enzymes). - Phenytoin induces UGT enzymes, which **increases retigabine metabolism and clearance**, leading to **decreased plasma concentrations**. - To maintain therapeutic levels and seizure control, the **dose of retigabine needs to be increased** when co-administered with phenytoin. *Stop the drug* - There is no clinical indication to completely **stop retigabine** simply because phenytoin is being added, as both can be used concurrently with dose adjustment. - Doing so might lead to a **loss of seizure control** if retigabine was providing effective seizure management. *Stop retigabine and start on carbamazepine* - This is an unnecessary and unindicated change; there is no medical reason to **switch from retigabine to carbamazepine** due to phenytoin addition. - Like phenytoin, **carbamazepine is also a strong enzyme inducer**, so similar drug interactions would occur. *Decrease the dose* - **Decreasing the dose** would be appropriate if phenytoin were an enzyme inhibitor, leading to higher retigabine levels. - Since phenytoin is an **enzyme inducer**, decreasing the dose would further reduce retigabine's therapeutic concentration, potentially leading to **breakthrough seizures**.

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**.

Question 1

A patient who was diagnosed with epilepsy was put on retigabine TDS. Now phenytoin is being added. Which of the following changes should be made to retigabine?

Accepted Answer

Increase the dose

Answer

Stop the drug

Answer

Stop retigabine and start on carbamazepine

Answer

Decrease the dose

Question 2

Which anti-asthma drug is avoided with erythromycin?

Accepted Answer

Theophylline

Answer

Ipratropium

Answer

Salbutamol

Answer

Terbutaline

Question 3

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 4

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 5

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 6

Which antibiotic potentiates the effect of neuromuscular blockers?

Accepted Answer

Aminoglycoside

Answer

Erythromycin

Answer

Nitrofurantoin

Answer

Co-trimoxazole

Question 7

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 8

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 9

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 10

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

Drug Interactions — MCQs

Drug Interactions — MCQs

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