Drug Interactions Practice Questions

Q: A patient with bronchial asthma is on theophylline. Which of the following drugs should be avoided for treating an upper respiratory tract infection in this patient?

Ciprofloxacin. ### Explanation The correct answer is **Ciprofloxacin**. **1. Why Ciprofloxacin is the correct answer:** Theophylline is a methylxanthine used in asthma that has a **narrow therapeutic index**. It is primarily metabolized by the hepatic enzyme **CYP1A2**. Ciprofloxacin is a potent **enzyme inhibitor** of CYP1A2. When co-administered, Ciprofloxacin inhibits the metabolism of theophylline, leading to significantly elevated plasma levels. This increases the risk of **theophylline toxicity**, which can manifest as severe nausea, vomiting, cardiac arrhythmias, and life-threatening seizures. **2. Why the other options are incorrect:** * **Amoxicillin and Ampicillin:** These are beta-lactam antibiotics (penicillins). Unlike certain macrolides or fluoroquinolones, penicillins do not significantly inhibit the Cytochrome P450 enzyme system. They do not interfere with theophylline clearance and are generally considered safe for treating infections in patients on theophylline. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Theophylline-Inhibitor" List:** Other drugs that commonly cause theophylline toxicity by inhibiting its metabolism include **Erythromycin, Clarithromycin, Cimetidine, and Allopurinol.** * **The "Theophylline-Inducer" List:** Conversely, drugs like **Rifampicin, Phenytoin, and Phenobarbitone**, as well as **smoking**, induce hepatic enzymes and *decrease* theophylline levels, potentially leading to therapeutic failure. * **Therapeutic Range:** The target plasma concentration for theophylline is **10–20 µg/mL**. Toxicity often begins when levels exceed 20 µg/mL. * **Management:** If a patient on theophylline requires a fluoroquinolone, **Levofloxacin** is a safer alternative as it has minimal effect on theophylline metabolism compared to Ciprofloxacin.

Q: Disulfiram-like reaction is not seen with which of the following drugs?

Amoxicillin. ### Explanation The **Disulfiram-like reaction** occurs when certain drugs inhibit the enzyme **Aldehyde Dehydrogenase (ALDH)**. When alcohol is consumed while taking these drugs, acetaldehyde (a toxic metabolite of ethanol) accumulates in the blood, leading to symptoms like flushing, tachycardia, palpitations, nausea, vomiting, and hypotension. **Why Amoxicillin is the Correct Answer:** * **Amoxicillin** is a penicillin-group antibiotic. It does not interfere with the metabolism of alcohol or inhibit ALDH. Therefore, it does not cause a disulfiram-like reaction. **Analysis of Incorrect Options:** * **Metronidazole:** This is the most classic example of a drug causing this reaction. Patients are strictly advised to avoid alcohol during and for 48 hours after treatment. * **Cefoperazone:** Several cephalosporins containing a **methylthiotetrazole (MTT) side chain** (e.g., Cefoperazone, Cefotetan, Cefamandole) inhibit ALDH and are well-known triggers for this reaction. * **Disulfiram:** This drug is intentionally used in aversion therapy for chronic alcoholism. It irreversibly inhibits ALDH to make alcohol consumption physically unpleasant. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Disulfiram-like drugs:** "**PM C**an't **G**o **T**o **S**chool" — **P**rocarbazine, **M**etronidazole, **C**ephalosporins (MTT chain), **G**riseofulvin, **T**olbutamide (1st gen Sulfonylureas), **S**ulfonamides. * **Mechanism:** Inhibition of Aldehyde Dehydrogenase $\rightarrow$ $\uparrow$ Acetaldehyde levels. * **Cephalosporins involved:** Cefoperazone, Cefotetan, Cefamandole, and Moxalactam. * **Other drugs:** Tinidazole, Chlorpropamide, and Nitrofurantoin.

Q: Which of the following drugs increases the activity of warfarin?

Erythromycin. **Explanation:** The correct answer is **Erythromycin**. **1. Why Erythromycin is correct:** Warfarin is an oral anticoagulant metabolized primarily by the hepatic cytochrome P450 enzyme system [1] (specifically **CYP2C9** [2]). Erythromycin is a potent **enzyme inhibitor**. By inhibiting these enzymes, erythromycin decreases the metabolism of warfarin, leading to increased plasma levels of the drug [1]. This enhances its anticoagulant effect, increases the International Normalized Ratio (INR), and significantly raises the risk of bleeding [1]. **2. Why the other options are incorrect:** * **Oral Contraceptive Pills (OCPs):** These generally decrease the effect of warfarin. Estrogens increase the synthesis of clotting factors (II, VII, IX, and X), which directly antagonizes the pharmacological action of warfarin. * **Griseofulvin:** This is a classic **enzyme inducer** [3]. It increases the synthesis of CYP450 enzymes, leading to faster metabolism of warfarin and a decrease in its clinical efficacy [3]. * **Phenytoin:** This is also a potent **enzyme inducer**. It stimulates the metabolism of warfarin, thereby reducing its anticoagulant activity [3]. (Note: Phenytoin has complex interactions as it is also highly protein-bound, but its primary long-term effect on warfarin is induction). **3. NEET-PG High-Yield Pearls:** * **Enzyme Inhibitors (Increase Warfarin activity):** "VITAMIN K" mnemonic – **V**erapamil, **I**soniazid, **T**rimethoprim-sulfamethoxazole, **A**miodarone, **M**etronidazole, **I**traconazole, **N**on-steroidal anti-inflammatory drugs (NSAIDs), **K**etoconazole (and Erythromycin/Cimetidine). * **Enzyme Inducers (Decrease Warfarin activity):** "GPRS Cell Phone" mnemonic – **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Broad-spectrum antibiotics** can also increase warfarin activity by killing gut flora that synthesize Vitamin K.

Q: An asthmatic patient on theophylline suddenly develops an infection. Which of the following antibiotics should be avoided to prevent side effects of theophylline?

Erythromycin. **Explanation:** The correct answer is **Erythromycin**. This question tests the concept of **Enzyme Inhibition** and its impact on drugs with a **Narrow Therapeutic Index (NTI)**. **1. Why Erythromycin is correct:** Theophylline is metabolized primarily by the hepatic cytochrome P450 enzyme system (specifically **CYP1A2** and **CYP3A4**). Erythromycin is a potent **enzyme inhibitor**. When co-administered, Erythromycin inhibits the metabolism of theophylline, leading to increased serum levels of the drug. Since theophylline has a narrow therapeutic window, this elevation can quickly lead to **theophylline toxicity**, manifesting as severe nausea, vomiting, palpitations, arrhythmias, and seizures. **2. Why other options are incorrect:** * **A & C (Ampicillin and Cephalexin):** These are Beta-lactam antibiotics. They are primarily excreted renally and do not significantly inhibit the CYP450 system. They have no clinically significant interaction with theophylline. * **D (Sparfloxacin):** While some fluoroquinolones (like Ciprofloxacin and Enoxacin) are known to inhibit theophylline metabolism, Sparfloxacin does not have a significant inhibitory effect on the CYP enzymes responsible for theophylline clearance. **3. NEET-PG High-Yield Pearls:** * **The "Cimetidine & Macrolide" Rule:** Always look for Enzyme Inhibitors like Cimetidine, Erythromycin, Clarithromycin, and Ciprofloxacin when a patient on Theophylline, Warfarin, or Phenytoin develops toxicity. * **Safe Macrolide:** **Azithromycin** is the macrolide of choice if an interaction must be avoided, as it does not inhibit CYP450 enzymes. * **Theophylline Toxicity:** The earliest sign is usually GI upset, but the most dangerous are cardiac arrhythmias and intractable seizures.

Q: Which drug does not interfere with antacids?

Azithromycin. ### Explanation The core pharmacological concept here is **chelation and pH-dependent absorption**. Antacids contain multivalent cations (e.g., $Al^{3+}$, $Mg^{2+}$, $Ca^{2+}$) which can bind to certain drugs, forming insoluble complexes that cannot be absorbed from the gastrointestinal tract. **Why Azithromycin is the Correct Answer:** Azithromycin is a macrolide antibiotic that does not form chelate complexes with multivalent cations. While its absorption can be slightly delayed by food, its total bioavailability is **not significantly affected** by the co-administration of antacids. Therefore, it does not require the strict spacing intervals necessary for other antibiotics. **Analysis of Incorrect Options:** * **Tetracycline:** This is the classic example of chelation. Tetracyclines bind strongly to $Ca^{2+}$ and $Mg^{2+}$ in antacids, forming non-absorbable chelates, leading to therapeutic failure. * **Norfloxacin:** Fluoroquinolones (like Norfloxacin and Ciprofloxacin) undergo significant chelation with antacids. This can reduce their bioavailability by up to 50–90%. * **Ranitidine:** While Ranitidine is used to reduce acid, taking it simultaneously with potent antacids (especially high-dose magnesium/aluminum hydroxides) can reduce its absorption by approximately 20-30% due to adsorption or pH changes. **NEET-PG High-Yield Pearls:** * **The "2-Hour Rule":** To avoid interactions, patients should be advised to take interacting drugs 2 hours before or 4–6 hours after antacids. * **Other drugs affected by antacids:** Iron salts, Digoxin, Ketoconazole (requires low pH for dissolution), and Phenytoin. * **Macrolide Exception:** Unlike Azithromycin, the absorption of **Erythromycin stearate** is significantly decreased by food and certain gastric conditions, though not primarily through chelation.

Q: Which of the following agents increases blood levels of theophylline?

Erythromycin. Theophylline is a methylxanthine bronchodilator with a **narrow therapeutic index**, primarily metabolized by the hepatic cytochrome P450 system (specifically **CYP1A2** and **CYP3A4**). Any drug that inhibits these enzymes will decrease the clearance of theophylline, leading to increased plasma levels and potential toxicity (e.g., seizures, arrhythmias). **1. Why Erythromycin is Correct:** Erythromycin is a potent **enzyme inhibitor** [1]. It binds to the CYP450 enzymes (specifically CYP3A4), forming an inactive complex [1]. This inhibits the metabolism of theophylline, significantly increasing its serum concentration and the risk of toxicity. **2. Why the Other Options are Incorrect:** * **Rifampicin:** This is a powerful **enzyme inducer** [2]. It increases the synthesis of CYP450 enzymes, thereby accelerating the metabolism of theophylline and *decreasing* its blood levels. * **Tobacco (Smoking):** Polycyclic aromatic hydrocarbons in cigarette smoke are potent **inducers of CYP1A2**. Chronic smokers require higher doses of theophylline because their clearance rate is significantly increased. * **Ethanol:** While acute large doses of alcohol can inhibit enzymes, chronic ethanol consumption generally acts as an **enzyme inducer** (CYP2E1), potentially lowering the levels of drugs metabolized by the liver. **High-Yield Clinical Pearls for NEET-PG:** * **Theophylline Toxicity:** Look for symptoms like persistent vomiting, tachycardia, and intractable seizures. * **Other Inhibitors (Increase Levels):** Ciprofloxacin, Cimetidine, Allopurinol, and Oral Contraceptive Pills (OCPs). * **Other Inducers (Decrease Levels):** Phenytoin, Phenobarbitone, and Carbamazepine. * **Therapeutic Range:** 10–20 µg/ml (Toxicity often starts >20 µg/ml).

Question 1

A patient with bronchial asthma is on theophylline. Which of the following drugs should be avoided for treating an upper respiratory tract infection in this patient?

Accepted Answer

Ciprofloxacin

Answer

Amoxicillin

Answer

Ampicillin

Answer

All of the above

Question 2

Disulfiram-like reaction is not seen with which of the following drugs?

Accepted Answer

Amoxicillin

Answer

Metronidazole

Answer

Cefoperazone

Answer

Disulfiram

Question 3

Which of the following drugs increases the activity of warfarin?

Accepted Answer

Erythromycin

Answer

Oral contraceptive pills (OCPs)

Answer

Griseofulvin

Answer

Phenytoin

Question 4

A 40-year-old male with a history of factor V Leiden deficiency and recurrent deep vein thrombosis is well controlled on warfarin. He presents with community-acquired pneumonia and is treated with erythromycin. Three days later, he develops bleeding and his INR is 8.0. Which of the following best explains why this bleeding occurred?

Accepted Answer

The erythromycin inhibited cytochrome P450

Answer

The erythromycin stimulated cytochrome P450

Answer

The causative agent of the pneumonia inhibited vitamin K utilization

Answer

The causative agent of the pneumonia stimulated vitamin K utilization

Question 5

Failure of oral contraceptives occurs when used with which of the following drugs?

Accepted Answer

Aspirin

Answer

Tetracycline

Answer

Phenytoin

Answer

Rifampicin

Question 6

An asthmatic patient on theophylline suddenly develops an infection. Which of the following antibiotics should be avoided to prevent side effects of theophylline?

Accepted Answer

Erythromycin

Answer

Ampicillin

Answer

Cephalexin

Answer

Sparfloxacin

Question 7

Antacids interfere with the absorption of which of the following drugs?

Accepted Answer

Azithromycin

Answer

Ketoconazole

Answer

Oxytetracycline

Answer

Ofloxacin

Question 8

Furosemide should not be administered with NSAIDS because they:

Accepted Answer

Inhibit prostacyclin synthesis

Answer

Prevent platelet aggregation

Answer

Decrease sodium reabsorption

Answer

Increase the secretion of furosemide in urine

Question 9

Which drug does not interfere with antacids?

Accepted Answer

Azithromycin

Answer

Tetracycline

Answer

Norfloxacin

Answer

Ranitidine

Question 10

Which of the following agents increases blood levels of theophylline?

Accepted Answer

Erythromycin

Answer

Rifampicin

Answer

Tobacco

Answer

Ethanol

Drug Interactions — MCQs

Drug Interactions — MCQs

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