Drug Interactions — MCQs

A 26-year-old female, currently taking phenytoin for idiopathic grand mal epilepsy, wishes to avoid pregnancy and is using oral contraceptive pills (OCPs). She is concerned that phenytoin may decrease the effectiveness of her OCPs. Which of the following is a suitable alternative anticonvulsant that is less likely to interact with OCPs?

Q79Medium

Which medication should be avoided in a patient with asthma who is currently on theophylline therapy, prior to dental extraction?

Q80Easy

Spina bifida is a known complication associated with the use of which medication?

Drug Interactions Indian Medical PG Practice Questions and MCQs

Question 71: Sparfloxacin and astemizole can cause which of the following?

A. Ventricular arrhythmia (Correct Answer)
B. Myopathy
C. Electrolyte imbalance
D. Nephropathy

Explanation: **Explanation:**The correct answer is **Ventricular arrhythmia**.**Mechanism of Action:**Both **Sparfloxacin** (a fluoroquinolone) and **Astemizole** (a second-generation antihistamine) share a common side effect: they cause **prolongation of the QT interval** on an ECG. They achieve this by blocking the delayed rectifier potassium channels ($I_{Kr}$) in the cardiac myocytes [3]. When two or more drugs that prolong the QT interval are administered together, their effects are additive. This significantly increases the risk of a specific, life-threatening polymorphic ventricular tachycardia known as **Torsades de Pointes (TdP)** [1, 2].**Analysis of Incorrect Options:** * **B. Myopathy:** This is a classic side effect of Statins (HMG-CoA reductase inhibitors) or Daptomycin, but it is not a characteristic interaction between fluoroquinolones and antihistamines.* **C. Electrolyte imbalance:** While hypokalemia or hypomagnesemia can *predispose* a patient to QT prolongation [1], these drugs themselves do not typically cause electrolyte shifts as their primary interaction.* **D. Nephropathy:** While some fluoroquinolones require dose adjustment in renal failure, they are not primarily known for causing synergistic nephrotoxicity when combined with antihistamines.**High-Yield Clinical Pearls for NEET-PG:** * **Astemizole and Terfenadine:** These antihistamines were largely withdrawn from the market because they caused TdP, especially when combined with CYP3A4 inhibitors (like Erythromycin or Ketoconazole) which increased their plasma levels.* **Fluoroquinolones:** Among this class, Sparfloxacin and Moxifloxacin have the highest propensity for QT prolongation.* **Other QT-prolonging drugs to remember:** Macrolides (Erythromycin), Antipsychotics (Haloperidol, Ziprasidone), Class IA and III Antiarrhythmics (Sotalol, Amiodarone), and Chloroquine.

Question 72: Which of the following drugs, when combined with erythromycin, may cause ventricular arrhythmias?

A. Tetracycline
B. Streptomycin
C. Ebastine
D. Cisapride (Correct Answer)

Explanation: **Explanation:** The correct answer is **Cisapride**. This interaction is a classic example of **enzyme inhibition** leading to life-threatening cardiac toxicity. **1. Why Cisapride is Correct:** Erythromycin is a potent inhibitor of the hepatic microsomal enzyme **CYP3A4**. Cisapride, a prokinetic agent, is primarily metabolized by this same enzyme. When co-administered, erythromycin inhibits the metabolism of cisapride, leading to significantly elevated plasma levels of the drug. High concentrations of cisapride block the delayed rectifier potassium channels ($I_{Kr}$) in the heart, resulting in **QT interval prolongation**, which can trigger **Torsades de Pointes (TdP)** and fatal ventricular arrhythmias. **2. Why Other Options are Incorrect:** * **Tetracycline:** While it is a bacteriostatic antibiotic like erythromycin, it does not significantly inhibit CYP3A4 or prolong the QT interval. * **Streptomycin:** This is an aminoglycoside. Its primary toxicities are ototoxicity and nephrotoxicity; it does not have a metabolic interaction with erythromycin involving the CYP system or cardiac rhythm. * **Ebastine:** Although some second-generation antihistamines (like Terfenadine and Astemizole) cause similar interactions with erythromycin, Ebastine has a much lower risk profile, and Cisapride is the more classic, high-yield "textbook" answer for this specific interaction. **Clinical Pearls for NEET-PG:** * **The "Dangerous Trio":** Erythromycin + (Cisapride / Terfenadine / Astemizole) = Risk of Torsades de Pointes. * **Macrolide Hierarchy:** Clarithromycin and Erythromycin are potent CYP3A4 inhibitors; **Azithromycin** is the safest macrolide as it does not inhibit CYP enzymes. * **Other QT Prolongers:** Fluoroquinolones, Class IA and III antiarrhythmics, and TCAs also increase the risk of arrhythmias when combined with CYP inhibitors.

Question 73: In a chronic alcoholic patient, which of the following drugs can be safely administered?

A. Cefamendole
B. Metronidazole
C. Chlorpropamide
D. Beclomethasone (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the **Disulfiram-like reaction**. Chronic alcoholics have high levels of acetaldehyde dehydrogenase (ALDH) activity, but certain drugs inhibit this enzyme. When alcohol is consumed with these drugs, acetaldehyde accumulates, leading to flushing, tachycardia, nausea, vomiting, and hypotension. **Why Beclomethasone is correct:** Beclomethasone is a **glucocorticoid** (corticosteroid) typically used in asthma or allergic rhinitis. It does not interfere with alcohol metabolism or the ALDH enzyme. Therefore, it does not cause a disulfiram-like reaction and can be safely administered to a chronic alcoholic patient. **Why the other options are incorrect:** * **Cefamandole:** This is a second-generation cephalosporin containing a **methylthiotetrazole (MTT) side chain**. This specific side chain inhibits ALDH, leading to a disulfiram-like reaction. (Other examples: Cefoperazone, Cefotetan). * **Metronidazole:** This is the classic example of a drug causing a disulfiram-like reaction. It inhibits ALDH and patients are strictly advised to avoid alcohol during and 48 hours after treatment. * **Chlorpropamide:** A first-generation sulfonylurea used in diabetes. It is well-known for causing "Sulfonylurea-induced flushing" when combined with alcohol due to ALDH inhibition. **NEET-PG High-Yield Pearls:** * **Mnemonic for Disulfiram-like drugs:** "**C**an **M**any **P**eople **G**et **D**runk?" (**C**ephalosporins with MTT chain, **M**etronidazole/Tinidazole, **P**rocarbazine, **G**riseofulvin, **D**isulfiram). * **Mechanism:** Inhibition of **Acetaldehyde Dehydrogenase**. * **Cephalosporins with MTT chain:** Cefamandole, Cefotetan, Cefoperazone (also cause hypoprothrombinemia/bleeding risk).

Question 74: Which of the following drugs can have an increased effect when given with heparin?

A. Narcotics
B. NSAIDS (Correct Answer)
C. OCPS
D. Salbutamol

Explanation: **Explanation:** The correct answer is **B. NSAIDs**. **Mechanism of Interaction:** Heparin is an anticoagulant that acts by activating Antithrombin III, which inhibits thrombin (Factor IIa) and Factor Xa. NSAIDs (Non-Steroidal Anti-inflammatory Drugs), such as aspirin or ibuprofen, interfere with primary hemostasis by inhibiting cyclooxygenase (COX) enzymes, thereby reducing **platelet aggregation**. When these two classes are combined, there is a **pharmacodynamic synergism**. While NSAIDs do not directly increase heparin’s plasma concentration, they significantly increase the **risk of bleeding** (the "effect" of anticoagulation) by impairing platelet function and potentially causing gastric mucosal erosions. **Analysis of Incorrect Options:** * **A. Narcotics (Opioids):** There is no significant direct interaction between narcotics and heparin. Narcotics primarily affect the CNS and GI motility. * **C. OCPs (Oral Contraceptive Pills):** Estrogen-containing OCPs are **pro-thrombotic** (increase clotting factors). They generally antagonize the therapeutic goal of heparin rather than increasing its effect. * **D. Salbutamol:** A beta-2 agonist used in asthma; it does not have a known clinical interaction with the coagulation cascade or heparin. **High-Yield Clinical Pearls for NEET-PG:** * **Antidote:** The specific antagonist for Heparin overdose is **Protamine Sulfate** (1 mg neutralizes ~100 units of heparin). * **Monitoring:** Unfractionated Heparin (UFH) is monitored using **aPTT**, whereas Low Molecular Weight Heparin (LMWH) usually does not require monitoring (except in pregnancy/obesity via Anti-Xa levels). * **HIT:** Watch for **Heparin-Induced Thrombocytopenia**, a paradoxical pro-thrombotic state caused by antibodies against Heparin-Platelet Factor 4 complexes.

Question 75: Phenobarbitone and warfarin interaction results in:

A. Increased absorption of warfarin
B. Increased metabolism of warfarin (Correct Answer)
C. Displacement of warfarin from the binding site
D. All of the above

Explanation: **Explanation:** The interaction between Phenobarbitone and Warfarin is a classic example of **Pharmacokinetic Drug Interaction** involving **Enzyme Induction**. **1. Why Option B is Correct:** Phenobarbitone is a potent inducer of the hepatic microsomal enzyme system, specifically the **Cytochrome P450 (CYP2C9)** isoenzyme. Warfarin is primarily metabolized by CYP2C9. When Phenobarbitone is co-administered, it increases the synthesis of these enzymes, leading to the **increased metabolism** of Warfarin. This results in decreased plasma concentrations of Warfarin, reducing its anticoagulant efficacy and lowering the International Normalized Ratio (INR). **2. Why Other Options are Incorrect:** * **Option A:** Phenobarbitone does not significantly affect the gastrointestinal absorption of Warfarin. * **Option C:** Displacement from binding sites (Protein Binding Displacement) is a mechanism associated with drugs like Sulfonamides or Salicylates, which can displace Warfarin from albumin, initially increasing its free fraction. Phenobarbitone does not act via this mechanism. **Clinical Pearls for NEET-PG:** * **Dose Adjustment:** If a patient on Warfarin starts Phenobarbitone, the dose of Warfarin must be **increased** to maintain the therapeutic INR. Conversely, if Phenobarbitone is stopped, the Warfarin dose must be **decreased** to prevent life-threatening bleeding. * **Other Potent Enzyme Inducers (GPRS Cell):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine. * **Enzyme Inhibitors (VITAMINS K):** **V**erapamil, **I**soniazid, **T**amoxifen, **A**miodarone, **M**etronidazole, **I**traconazole, **N**elnavir, **S**ulfonamides, **K**etoconazole (these would *increase* Warfarin toxicity).

Question 76: A diabetic female on isoniazid and rifampicin for tuberculosis suffers from deep vein thrombosis. She is started on warfarin but her prothrombin time is not raised. What is the next best step in management?

A. Use low molecular weight heparin (Correct Answer)
B. Long term heparin therapy
C. Switch ethambutol for rifampicin
D. Replace warfarin with acenocoumarol

Explanation: ### Explanation **Concept: Enzyme Induction and Drug Interactions** The core issue in this clinical scenario is a significant drug interaction. **Rifampicin** is a potent **microsomal enzyme inducer** (specifically CYP3A4 and other P450 enzymes). It increases the metabolism of **Warfarin**, thereby reducing its plasma concentration and therapeutic efficacy. This explains why the patient’s Prothrombin Time (PT/INR) remains unchanged despite being on warfarin. **Why Option A is Correct:** In a patient with active Deep Vein Thrombosis (DVT) where oral anticoagulation is failing due to enzyme induction, the immediate priority is to achieve therapeutic anticoagulation to prevent pulmonary embolism. **Low Molecular Weight Heparin (LMWH)** is the drug of choice because its metabolism is **not dependent on the CYP450 system**. It provides a predictable anticoagulant effect that is unaffected by Rifampicin. **Why Other Options are Incorrect:** * **Option B:** Long-term unfractionated heparin is impractical due to the need for continuous monitoring (aPTT) and the risk of Heparin-Induced Thrombocytopenia (HIT) and osteoporosis. * **Option C:** Rifampicin is a "first-line" bactericidal drug essential for TB treatment. Switching it for Ethambutol (a bacteriostatic drug) would weaken the TB regimen and increase the risk of treatment failure or resistance. * **Option D:** Acenocoumarol is also a vitamin K antagonist metabolized by the liver; it would face the same enzyme-induction issues as warfarin. **Clinical Pearls for NEET-PG:** * **Rifampicin** is the most potent inducer of the CYP450 system. * **Isoniazid**, conversely, is an enzyme **inhibitor**, but in this combination, the inducing effect of Rifampicin usually predominates. * **Alternative:** If oral anticoagulation is mandatory, the dose of warfarin may need to be increased by 2–3 times, but switching to LMWH or Fondaparinux is safer in the acute phase. * **LMWH** (e.g., Enoxaparin) acts primarily by inhibiting Factor Xa and does not require routine PT/INR monitoring.

Question 77: Carbamazepine has drug interaction with all of the following except?

A. Erythromycin
B. Phenytoin
C. Doxycycline (Correct Answer)
D. Barbiturates

Explanation: **Explanation:** The core concept behind this question is the **Cytochrome P450 (CYP450) enzyme system**, specifically the role of Carbamazepine as a potent **enzyme inducer**. **Why Doxycycline is the correct answer:** Carbamazepine induces CYP3A4, which increases the metabolism of several drugs. While Carbamazepine *does* decrease the half-life of Doxycycline (reducing its efficacy), the question asks which drug Carbamazepine has an interaction *with*—implying a clinically significant bidirectional or inhibitory interaction that alters Carbamazepine levels. In standard pharmacological teaching for NEET-PG, Doxycycline is the "odd one out" because it does not significantly alter the plasma concentration of Carbamazepine. **Analysis of Incorrect Options:** * **Erythromycin:** This is a potent **CYP3A4 inhibitor**. It inhibits the metabolism of Carbamazepine, leading to toxic levels of the antiepileptic drug (ataxia, drowsiness, diplopia). * **Phenytoin & Barbiturates:** Both are potent **enzyme inducers**. They induce the metabolism of Carbamazepine (and vice versa), leading to decreased plasma concentrations of the drugs. This necessitates dosage adjustments to maintain therapeutic efficacy. **High-Yield Clinical Pearls for NEET-PG:** 1. **Auto-induction:** Carbamazepine is unique because it induces its own metabolism (auto-induction), typically starting after 3–5 days of therapy and completing within 3–4 weeks. 2. **The "G-PACMAN" Mnemonic (Inducers):** **G**riseofulvin, **P**henytoin, **A**lcohol (chronic), **C**arbamazepine, **M**odafinil, **A**minoglutethimide, **N**evirapine. 3. **Drug of Choice:** Carbamazepine remains the drug of choice for **Trigeminal Neuralgia**. 4. **Side Effects:** Watch for SIADH (hyponatremia) and Stevens-Johnson Syndrome (especially in patients with HLA-B*1502 allele).

Question 78: A 26-year-old female, currently taking phenytoin for idiopathic grand mal epilepsy, wishes to avoid pregnancy and is using oral contraceptive pills (OCPs). She is concerned that phenytoin may decrease the effectiveness of her OCPs. Which of the following is a suitable alternative anticonvulsant that is less likely to interact with OCPs?

A. Carbamazepine
B. Lamotrigine (Correct Answer)
C. Topiramate
D. Phenobarbitone

Explanation: ### Explanation **Core Concept: Enzyme Induction and OCP Failure** The primary concern in this clinical scenario is the induction of hepatic microsomal enzymes (specifically **CYP3A4**). Most traditional anticonvulsants are potent enzyme inducers that accelerate the metabolism of estrogen and progesterone components in OCPs, leading to decreased plasma levels and potential contraceptive failure [1]. **Why Lamotrigine is Correct:** **Lamotrigine** is the preferred choice here because it is **not a clinically significant hepatic enzyme inducer**. Unlike phenytoin, it does not accelerate the breakdown of OCPs [2]. * *Note:* While OCPs can actually decrease the plasma levels of Lamotrigine (requiring dose adjustment), Lamotrigine itself does not compromise the efficacy of the OCP. Other safe alternatives for women on OCPs include **Levetiracetam, Valproate, and Gabapentin.** **Why Other Options are Incorrect:** * **A. Carbamazepine:** A potent inducer of CYP1A2, 2C9, and 3A4. It significantly increases OCP metabolism. * **C. Topiramate:** While it has multiple mechanisms, at doses >200 mg/day, it acts as a CYP3A4 inducer and can reduce OCP effectiveness. * **D. Phenobarbitone:** A classic, broad-spectrum enzyme inducer that significantly lowers the serum concentration of contraceptive steroids. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Inducers:** "**G**uiness **P**is **S**strong **C**ombat **R**ifle" (**G**riseofulvin, **P**henytoin, **S**moking, **C**arbamazepine, **R**ifampicin, **P**henobarbitone). * **Teratogenicity:** Phenytoin is associated with **Fetal Hydantoin Syndrome** (cleft lip/palate, digital hypoplasia). Valproate is the most teratogenic (neural tube defects). * **Drug of Choice:** Lamotrigine and Levetiracetam are often preferred in pregnancy and women of childbearing age due to a better safety profile and fewer drug interactions [1].

Question 79: Which medication should be avoided in a patient with asthma who is currently on theophylline therapy, prior to dental extraction?

A. Lignocaine
B. Azithromycin
C. Ciprofloxacin (Correct Answer)
D. Metronidazole

Explanation: **Explanation:** The correct answer is **Ciprofloxacin**. This question tests the knowledge of **Cytochrome P450 (CYP450) enzyme inhibition** and its impact on drugs with a narrow therapeutic index, like Theophylline. **1. Why Ciprofloxacin is correct:** Theophylline is primarily metabolized by the hepatic enzyme **CYP1A2**. Ciprofloxacin is a potent inhibitor of CYP1A2. When co-administered, Ciprofloxacin inhibits the metabolism of Theophylline, leading to significantly elevated serum levels. Since Theophylline has a **narrow therapeutic index**, this interaction can result in toxicity, manifesting as severe nausea, vomiting, cardiac arrhythmias, and seizures. **2. Why the other options are incorrect:** * **Lignocaine:** While Lignocaine is also metabolized by CYP enzymes, it does not significantly inhibit the metabolism of Theophylline. It is the standard local anesthetic used in dental extractions. * **Azithromycin:** Unlike other macrolides (such as Erythromycin or Clarithromycin), Azithromycin does **not** significantly inhibit CYP450 enzymes and is generally considered safe to use with Theophylline. * **Metronidazole:** This drug primarily inhibits CYP2C9 (affecting Warfarin). It does not have a clinically significant interaction with Theophylline metabolism. **Clinical Pearls for NEET-PG:** * **Theophylline Toxicity:** Remember the "Three C's" of enzyme inhibitors that increase Theophylline levels: **C**iprofloxacin, **C**imetidine, and **C**larithromycin/Erythromycin. * **Narrow Therapeutic Index Drugs:** Always monitor for interactions when a patient is on Theophylline, Warfarin, Digoxin, or Lithium. * **Safe Alternative:** If an antibiotic is needed for a patient on Theophylline, **Amoxicillin** or **Azithromycin** are preferred choices.

Question 80: Spina bifida is a known complication associated with the use of which medication?

A. Carbamazepine (Correct Answer)
B. Clozapine
C. Lithium
D. Olanzapine

Explanation: **Explanation:** The correct answer is **Carbamazepine**. **1. Why Carbamazepine is Correct:** Carbamazepine is a classic enzyme-inducing anticonvulsant. It is a known human teratogen associated with a 1% risk of **Neural Tube Defects (NTDs)**, specifically **Spina Bifida**. The underlying mechanism involves the drug’s interference with **folate metabolism** and the production of reactive epoxide metabolites. When taken during the first trimester (the period of organogenesis), it inhibits the proper closure of the neural tube. **2. Why the Other Options are Incorrect:** * **Clozapine & Olanzapine:** These are atypical antipsychotics. They are generally not associated with structural malformations like Spina Bifida. Their primary concern in pregnancy is metabolic (e.g., gestational diabetes and excessive weight gain). * **Lithium:** This mood stabilizer is classically associated with **Ebstein’s Anomaly**, a congenital cardiac defect involving the tricuspid valve, rather than neural tube defects. **3. High-Yield Clinical Pearls for NEET-PG:** * **Valproate vs. Carbamazepine:** While both cause Spina Bifida, **Sodium Valproate** carries the highest risk (approx. 2-5%) and is the most common cause of drug-induced NTDs. * **Prevention:** To reduce the risk of NTDs in women taking antiepileptic drugs (AEDs), high-dose **Folic Acid (4-5 mg/day)** supplementation is recommended starting before conception. * **Fetal Hydantoin Syndrome:** Associated with **Phenytoin**, characterized by craniofacial anomalies, hypoplastic nails, and phalanges. * **Safe AEDs in Pregnancy:** Levetiracetam and Lamotrigine are currently considered to have the lowest teratogenic potential.

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Clinically Significant Drug Interactions

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Management of Drug Interactions

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