Drug Interactions Practice Questions

Q: Tricyclic antidepressants can alter the oral absorption of many drugs by which of the following mechanisms?

Altering gut motility. **Explanation:** **Mechanism of Action (Correct Answer):** Tricyclic antidepressants (TCAs), such as Amitriptyline and Imipramine, possess significant **antimuscarinic (anticholinergic) properties**. By blocking muscarinic receptors in the gastrointestinal tract, they decrease parasympathetic stimulation, leading to **delayed gastric emptying and reduced intestinal motility**. Since the rate of drug absorption is highly dependent on gastric emptying time and intestinal transit, TCAs can significantly alter the pharmacokinetics of co-administered drugs. For instance, delayed emptying may slow the absorption of drugs like paracetamol, or conversely, increase the total absorption of drugs that are poorly soluble by allowing them more time to dissolve in the gut. **Analysis of Incorrect Options:** * **Option A (Complexing):** This refers to chemical interactions (chelation) in the lumen, typical of drugs like **Cholestyramine** or **Antacids** (which bind to Tetracyclines/Fluoroquinolones), not TCAs. * **Option C (Altering gut flora):** This is a mechanism associated with **broad-spectrum antibiotics**, which can interfere with the enterohepatic circulation of drugs like oral contraceptives. * **Option D (Damaging gut mucosa):** This is characteristic of cytotoxic **chemotherapeutic agents** (e.g., Methotrexate) or chronic NSAID use, leading to malabsorption syndromes. **NEET-PG High-Yield Pearls:** * **Anticholinergic Toxidrome:** Remember the mnemonic "Blind as a bat, Mad as a hatter, Red as a beet, Hot as a hare, **Bowel and bladder lose their tone**." * **Other drugs causing decreased motility:** Opioids, Atropine, and Phenothiazines. * **Prokinetic agents:** Drugs like Metoclopramide *increase* motility and can speed up the absorption of other drugs.

Q: Concomitant administration of clonazepam with which of the following antiepileptic drugs can precipitate absence status?

Sodium valproate. **Explanation:** The correct answer is **Sodium Valproate**. **Mechanism of Interaction:** The precipitation of **absence status (non-convulsive status epilepticus)** is a well-documented, though rare, idiosyncratic drug interaction between **clonazepam** and **sodium valproate**. While both drugs are individually effective in treating absence seizures, their concomitant use can paradoxically trigger prolonged absence attacks. The exact pathophysiology is not fully understood, but it is hypothesized to involve complex alterations in GABAergic neurotransmission within the thalamocortical pathways, which are responsible for the 3-Hz spike-and-wave discharges characteristic of absence epilepsy. **Analysis of Incorrect Options:** * **Phenobarbitone:** This is a barbiturate that increases the duration of GABA-A channel opening. While it causes sedation when used with benzodiazepines, it does not specifically precipitate absence status. * **Carbamazepine & Phenytoin:** These are sodium channel blockers. They are generally **contraindicated** in absence seizures because they can aggravate them, but they do not have a specific "status-precipitating" interaction with clonazepam. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Sodium Valproate is the DOC for generalized tonic-clonic seizures (GTCS) and myoclonic seizures. Ethosuximide is the DOC for pure absence seizures. * **Paradoxical Effect:** Always remember the "Valproate + Clonazepam = Absence Status" triad for pharmacology MCQs. * **Contraindication:** Avoid Carbamazepine, Phenytoin, and Vigabatrin in patients with absence or myoclonic seizures as they can worsen the condition. * **Valproate Side Effects:** Hepatotoxicity (highest risk <2 years old), Teratogenicity (Neural tube defects), and Weight gain.

Q: Which of the following drugs does NOT interfere with the effectiveness of oral contraceptive pills?

Aspirin. **Explanation:** The effectiveness of Oral Contraceptive Pills (OCPs) is primarily compromised by drugs that induce hepatic microsomal enzymes (Cytochrome P450). OCPs contain estrogen (ethinyl estradiol) and progestins, which are metabolized in the liver. When an enzyme inducer is co-administered, the metabolism of these hormones increases, leading to sub-therapeutic serum levels and potential contraceptive failure. **Why Aspirin is the Correct Answer:** Aspirin is a Non-Steroidal Anti-Inflammatory Drug (NSAID) that acts by irreversibly inhibiting the cyclooxygenase (COX) enzymes. It does **not** induce or inhibit hepatic microsomal enzymes. Therefore, it has no significant pharmacokinetic interaction with OCPs and does not reduce their contraceptive efficacy. **Why the Other Options are Incorrect:** * **Rifampicin:** This is the most potent inducer of the CYP3A4 enzyme. It significantly increases the clearance of estrogen, making it the most common cause of drug-induced OCP failure. * **Phenobarbital & Primidone:** Both are barbiturate antiepileptics (Primidone is metabolized to phenobarbital). They are classic hepatic enzyme inducers that stimulate the metabolism of steroid hormones, reducing OCP effectiveness. **High-Yield Clinical Pearls for NEET-PG:** * **Potent Enzyme Inducers (Mnemonic: GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbital. All of these can cause OCP failure. * **Antibiotic Myth:** While Rifampicin is a proven inducer, most broad-spectrum antibiotics (like Amoxicillin) do not significantly reduce OCP efficacy in most women, though they were historically thought to interfere via enterohepatic circulation. * **Clinical Advice:** Patients on long-term enzyme inducers should be advised to use an alternative form of contraception (e.g., IUD) or a higher dose of estrogen (at least 50μg).

Q: In a patient with chronic asthma being treated with theophylline, which of the following medications should not be used to treat an accompanying upper respiratory tract infection?

Erythromycin. **Explanation:** The correct answer is **Erythromycin** because of its significant inhibitory effect on the hepatic cytochrome P450 enzyme system, specifically the **CYP1A2 and CYP3A4** isoenzymes. **1. Why Erythromycin is the Correct Answer:** Theophylline is a methylxanthine with a **narrow therapeutic index** (10–20 µg/mL), meaning small changes in its serum concentration can lead to severe toxicity. It is primarily metabolized by the liver (CYP1A2). Erythromycin (a macrolide) inhibits these enzymes, leading to a decrease in theophylline clearance. This results in elevated theophylline levels, increasing the risk of toxicity, which manifests as severe nausea, vomiting, cardiac arrhythmias, and seizures. **2. Why Other Options are Incorrect:** * **Ampicillin (Penicillin) & Cephalexin (Cephalosporin):** These are beta-lactam antibiotics. They are primarily excreted renally and do not significantly interfere with the hepatic microsomal enzyme system. They do not alter theophylline metabolism and are generally considered safe to use concurrently. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Inhibitors (Theophylline toxicity):** Remember the mnemonic **"VITAMIN K"** or **"Cimetidine, Ciprofloxacin, Erythromycin, and Clarithromycin"** as key drugs that increase theophylline levels. * **Enzyme Inducers (Sub-therapeutic levels):** Smoking, Rifampicin, Phenytoin, and Phenobarbitone increase theophylline clearance, necessitating a dose increase. * **Macrolide Exception:** While Erythromycin and Clarithromycin are potent inhibitors, **Azithromycin** does not significantly inhibit CYP enzymes and is a safer alternative in patients on theophylline. * **Quinolone Alert:** Ciprofloxacin is another high-yield drug that causes the same interaction as Erythromycin.

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: Which of the following proton pump inhibitors has enzyme inhibitory activity?

Omeprazole. **Explanation:** **1. Why Omeprazole is the Correct Answer:** Omeprazole is a potent inhibitor of the hepatic microsomal enzyme system, specifically **CYP2C19** and **CYP3A4**. By inhibiting these enzymes, it reduces the metabolism of several drugs, leading to increased plasma concentrations and potential toxicity. This is a classic pharmacological property of the first-generation PPI, Omeprazole, which distinguishes it from newer agents in the class. **2. Why the Other Options are Incorrect:** * **Pantoprazole:** Known for having the **least potential** for drug-drug interactions. It has lower affinity for cytochrome P450 enzymes, making it the preferred PPI for patients on multiple medications (polypharmacy). * **Rabeprazole:** Primarily metabolized via a non-enzymatic pathway (reduction) with minor CYP involvement, resulting in minimal enzyme inhibition. * **Lansoprazole:** While it is metabolized by CYP enzymes, its inhibitory effect on the metabolism of other drugs is clinically insignificant compared to Omeprazole. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Clopidogrel Interaction:** This is the most high-yield clinical application. Clopidogrel is a prodrug activated by **CYP2C19**. Because Omeprazole inhibits this enzyme, it prevents the activation of Clopidogrel, potentially increasing the risk of cardiovascular events (thrombosis). * **Other Interactions:** Omeprazole increases the half-life of **Warfarin, Phenytoin, and Diazepam**. * **Mechanism of Action:** All PPIs are prodrugs that irreversibly inhibit the **H+/K+ ATPase pump** (proton pump) in gastric parietal cells. * **Fastest Acting PPI:** Rabeprazole is often cited as having the fastest onset of action among the group.

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.

Question 1

Tricyclic antidepressants can alter the oral absorption of many drugs by which of the following mechanisms?

Accepted Answer

Altering gut motility

Answer

Complexing with other drugs in the intestinal lumen

Answer

Altering gut flora

Answer

Damaging gut mucosa

Question 2

Concomitant administration of clonazepam with which of the following antiepileptic drugs can precipitate absence status?

Accepted Answer

Sodium valproate

Answer

Phenobarbitone

Answer

Carbamazepine

Answer

Phenytoin

Question 3

Which of the following drug classes, if combined with Rivastigmine, decreases its efficacy?

Accepted Answer

Tricyclic antidepressants

Answer

Selective serotonin reuptake inhibitors

Answer

Reversible inhibitor of MAO-A

Answer

Atypical antidepressants

Question 4

Which of the following drugs does NOT interfere with the effectiveness of oral contraceptive pills?

Accepted Answer

Aspirin

Answer

Phenobarbital

Answer

Rifampicin

Answer

Primidone

Question 5

In a patient with chronic asthma being treated with theophylline, which of the following medications should not be used to treat an accompanying upper respiratory tract infection?

Accepted Answer

Erythromycin

Answer

Ampicillin

Answer

Cephalexin

Answer

All of the above

Question 6

Which of the following drugs may cause hypertensive crisis in a patient on monoamine oxidase inhibitor therapy?

Accepted Answer

Tyramine

Answer

Guanethidine

Answer

Phenobarbitone

Answer

Norepinephrine

Question 7

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 8

Which of the following proton pump inhibitors has enzyme inhibitory activity?

Accepted Answer

Omeprazole

Answer

Rabeprazole

Answer

Lansoprazole

Answer

Pantoprazole

Question 9

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

Accepted Answer

Amoxicillin

Answer

Metronidazole

Answer

Cefoperazone

Answer

Disulfiram

Question 10

Which of the following drugs increases the activity of warfarin?

Accepted Answer

Erythromycin

Answer

Oral contraceptive pills (OCPs)

Answer

Griseofulvin

Answer

Phenytoin

Drug Interactions — MCQs

Drug Interactions — MCQs

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