Drug Interactions Practice Questions

Q: Which of the following is a P-Glycoprotein inhibitor?

Ketoconazole. **Explanation:** **P-glycoprotein (P-gp)** is an ATP-dependent efflux transporter found in the gut lining, blood-brain barrier, and renal tubules. It pumps drugs out of cells, thereby limiting their absorption and increasing their elimination. **Why Ketoconazole is Correct:** **Ketoconazole** is a potent **inhibitor** of both CYP3A4 and P-glycoprotein. By inhibiting P-gp, it reduces the efflux of substrate drugs (like Digoxin or Cyclosporine), leading to increased plasma concentrations and potential toxicity. Most drugs that inhibit CYP3A4 also tend to inhibit P-gp. **Why the Other Options are Incorrect:** * **Rifampicin:** This is a classic **inducer** of P-gp. It increases the expression of the transporter, leading to decreased plasma levels of P-gp substrates. * **Phenytoin:** Similar to Rifampicin, Phenytoin is a broad-spectrum microsomal enzyme **inducer** and also induces P-gp. * **Griseofulvin:** This antifungal agent acts as an enzyme **inducer**. It does not inhibit P-gp. **High-Yield Clinical Pearls for NEET-PG:** * **Key P-gp Inhibitors:** Verapamil (most potent), Quinidine, Ketoconazole, Itraconazole, Erythromycin, and Amiodarone. * **Key P-gp Inducers:** Rifampicin, St. John’s Wort, Phenytoin, and Carbamazepine. * **Important Substrate:** **Digoxin** is the prototype P-gp substrate. Co-administration of Verapamil or Ketoconazole with Digoxin can lead to **Digoxin toxicity** due to inhibited renal clearance and increased intestinal absorption. * **Location:** P-gp is encoded by the **MDR1** (Multi-Drug Resistance) gene. Overexpression of this gene in cancer cells is a major cause of resistance to chemotherapy (e.g., Vincristine, Doxorubicin).

Q: The anticoagulant effect of warfarin is increased by all of the following drugs, EXCEPT:

Phytonadione. **Explanation:** The anticoagulant effect of **Warfarin** is highly sensitive to drug interactions. Warfarin acts by inhibiting the enzyme **Vitamin K Epoxide Reductase (VKORC1)**, which prevents the recycling of Vitamin K, thereby inhibiting the synthesis of clotting factors II, VII, IX, and X. **Why Phytonadione is the correct answer:** **Phytonadione (Vitamin K1)** is the physiological antagonist to Warfarin. By providing an exogenous source of Vitamin K, it bypasses the inhibition caused by Warfarin and promotes the synthesis of clotting factors. Therefore, it **decreases** (reverses) the anticoagulant effect of Warfarin rather than increasing it. It is the specific antidote for Warfarin overdose. **Analysis of Incorrect Options:** * **Cimetidine:** An H2-blocker that acts as a potent **Microsomal Enzyme Inhibitor** (CYP450). It decreases the metabolism of Warfarin, leading to increased plasma levels and an enhanced anticoagulant effect (increased risk of bleeding). * **Amiodarone:** A Class III antiarrhythmic that inhibits CYP2C9, the primary enzyme responsible for metabolizing the more potent S-isomer of Warfarin. This significantly increases Warfarin's effect. * **Phenylbutazone:** An NSAID that increases Warfarin's effect through two mechanisms: it **displaces Warfarin from plasma albumin** (increasing the free drug fraction) and inhibits its metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Inducers (Decrease Warfarin effect):** Rifampicin, Phenytoin, Phenobarbitone, Carbamazepine, and Chronic Alcoholism. * **Enzyme Inhibitors (Increase Warfarin effect):** Erythromycin, Ketoconazole, Ciprofloxacin, and Metronidazole. * **Monitoring:** Warfarin therapy is monitored using **PT/INR** (Target: 2.0–3.0). * **Broad-spectrum antibiotics** can also increase Warfarin's effect by killing Vitamin K-producing gut flora.

Q: Oral contraceptive pills' efficiency is reduced by the simultaneous use of which of the following medications?

Rifampicin and Carbamazepine. **Explanation:** The efficacy of Oral Contraceptive Pills (OCPs) is primarily compromised by drugs that act as **Microsomal Enzyme Inducers**. **1. Why Option A is Correct:** Both **Rifampicin** and **Carbamazepine** are potent inducers of the Cytochrome P450 (CYP3A4) enzyme system in the liver. When these enzymes are induced, the metabolic breakdown of estrogen and progesterone components of the OCP is significantly accelerated. This leads to sub-therapeutic plasma levels of the hormones, failing to suppress ovulation and resulting in **contraceptive failure**. Rifampicin is often cited as the most potent inducer in this context. **2. Analysis of Incorrect Options:** * **Option B:** While Rifampicin does reduce OCP efficacy, this option is incomplete because Carbamazepine (in Option A) also shares this clinically significant interaction. * **Option C:** Tricyclic Antidepressants (TCAs) do not typically induce hepatic enzymes; in fact, OCPs may actually inhibit the metabolism of TCAs, leading to increased TCA toxicity rather than reduced OCP efficacy. * **Option D:** Propranolol is a non-selective beta-blocker and does not induce microsomal enzymes. Like TCAs, its clearance may be decreased by OCPs, but it does not cause OCP failure. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Enzyme Inducers:** "GPRS Cell Phone" (**G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone). * **Clinical Advice:** Patients on OCPs starting an enzyme inducer should be advised to use a backup barrier method (e.g., condoms) or switch to a non-hormonal method like an IUD. * **Antibiotic Myth:** Apart from Rifampicin (and possibly Griseofulvin), most routine antibiotics (like Amoxicillin or Doxycycline) do not significantly reduce OCP efficacy in clinical practice, despite historical concerns.

Q: Which of the following does not affect the metabolism of oral contraceptive pills?

Penicillin. **Explanation:** The efficacy of Oral Contraceptive Pills (OCPs) is primarily dependent on their metabolism by the hepatic **Cytochrome P450 (CYP450)** enzyme system. **1. Why Penicillin is the correct answer:** Penicillin is a cell wall synthesis inhibitor and does not significantly induce or inhibit hepatic microsomal enzymes. While some broad-spectrum antibiotics (like Tetracyclines or Ampicillin) were historically thought to reduce OCP efficacy by interfering with enterohepatic circulation (gut flora changes), clinical studies have shown that **Penicillin does not decrease the plasma concentration of OCPs** to a level that causes contraceptive failure. **2. Why the other options are incorrect:** * **Phenytoin (Option A):** A potent **CYP3A4 inducer**. It increases the metabolism of estrogen and progesterone, leading to decreased plasma levels and potential contraceptive failure. * **Griseofulvin (Option B):** An antifungal agent known to be a **microsomal enzyme inducer**. It accelerates the breakdown of OCPs. * **Primidone (Option C):** A barbiturate derivative (pro-drug of Phenobarbital) and a strong **enzyme inducer**. It significantly lowers the efficacy of hormonal contraceptives. **Clinical Pearls for NEET-PG:** * **Rifampicin** is the most potent enzyme inducer and the most common cause of OCP failure among antibiotics. * **Mnemonic for Enzyme Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * When a patient is on enzyme-inducing AEDs (like Phenytoin), the recommended strategy is to use an alternative method (IUCD) or a higher dose of estrogen (at least 50μg).

Q: Disulfiram-like interaction with alcohol is seen with all of the following drugs except?

Satranidazole. **Explanation:** The **Disulfiram-like reaction** occurs when a drug inhibits the enzyme **Aldehyde Dehydrogenase (ALDH)**. This leads to the accumulation of acetaldehyde in the blood after alcohol consumption, causing symptoms like flushing, tachycardia, palpitations, nausea, and hypotension. **Why Satranidazole is the correct answer:** While most 5-nitroimidazoles (like Metronidazole and Tinidazole) are notorious for causing disulfiram-like reactions, **Satranidazole** is a newer imidazole derivative with a modified side chain. It **does not inhibit aldehyde dehydrogenase** and therefore does not produce a disulfiram-like reaction when taken with alcohol. This makes it a clinically distinct option in the treatment of amoebiasis. **Analysis of incorrect options:** * **Metronidazole:** The classic example of a drug causing this reaction. Patients are strictly advised to avoid alcohol during and for 48 hours after therapy. * **Cefoperazone:** Several cephalosporins containing the **methylthiotetrazole (MTT) side chain** (e.g., Cefoperazone, Cefotetan, Cefamandole) inhibit ALDH and cause this interaction. * **Griseofulvin:** This antifungal agent is well-known to potentiate the effects of alcohol and can trigger a disulfiram-like syndrome in susceptible individuals. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Disulfiram-like drugs:** "**C**an **G**et **M**any **P**roblems **S**oon" (**C**ephalosporins [MTT chain], **G**riseofulvin, **M**etronidazole/Tinidazole, **P**rocarbazine, **S**ulfonylureas [1st Gen like Chlorpropamide]). * **Chlorpropamide** (1st Gen Sulfonylurea) has the highest incidence among antidiabetics. * **Satranidazole** has a longer half-life and better tolerability compared to Metronidazole, specifically lacking the metallic taste and the disulfiram-like potential.

Q: Which of the following is not an inducer of CYP3A4?

Erythromycin. The Cytochrome P450 (CYP) enzyme system is the primary pathway for hepatic drug metabolism. CYP3A4 is the most abundant isoform, responsible for metabolizing nearly 50% of all clinically used drugs. Why Erythromycin is the correct answer: Erythromycin is a potent inhibitor of CYP3A4, not an inducer. It binds covalently to the iron atom of the heme group in the enzyme, forming an inactive complex [1]. This inhibition leads to decreased metabolism and increased toxicity of co-administered substrates like Theophylline, Warfarin, and Statins [2]. Why the other options are incorrect: * A. Barbiturates (e.g., Phenobarbital): These are classic, broad-spectrum microsomal enzyme inducers [1]. They increase the synthesis of CYP enzymes (3A4, 2C9) by activating the Constitutive Androstane Receptor (CAR). * B. Glucocorticoids: Drugs like Dexamethasone act as inducers of CYP3A4 by binding to the Pregnane X Receptor (PXR), which upregulates enzyme transcription [2]. * C. Phenytoin: A potent inducer of multiple CYP isoforms, including 3A4 [1]. This often leads to significant drug-drug interactions, such as reducing the efficacy of oral contraceptives. High-Yield Clinical Pearls for NEET-PG: * Mnemonic for Inducers (GPRS Cell Phone): Griseofulvin, Phenytoin, Rifampicin, Smoking, Carbamazepine, Phenobarbital. (Note: St. John’s Wort is also a potent inducer [2]). * Mnemonic for Inhibitors (VITAMIN K): Valproate, Isoniazid, Terefenadine (historical), Amiodarone, Macrolides (except Azithromycin), Indinavir, Non-DHP CCBs (Verapamil/Diltiazem), Ketoconazole (and Cimetidine/Grapefruit juice [2]). * Key Distinction: Azithromycin is the only common macrolide that does not significantly inhibit CYP3A4.

Q: A disulfiram-like reaction is seen with which of the following medications?

Metronidazole. ### Explanation **Correct Option: A (Metronidazole)** The **disulfiram-like reaction** occurs when certain drugs inhibit the enzyme **aldehyde dehydrogenase (ALDH)**. Under normal circumstances, alcohol is metabolized into acetaldehyde, which ALDH then converts into harmless acetic acid. When ALDH is inhibited, **acetaldehyde accumulates** in the bloodstream, leading to symptoms such as flushing, tachycardia, palpitations, nausea, vomiting, and hypotension. **Metronidazole** is the classic antibiotic associated with this reaction; patients are strictly advised to avoid alcohol during treatment and for at least 48–72 hours after the last dose. **Analysis of Incorrect Options:** * **B. Benzodiazepines:** These are GABA-A receptor agonists used to treat alcohol withdrawal. They do not interfere with alcohol metabolism but have a synergistic CNS depressant effect if combined with alcohol. * **C. Fluoxetine:** An SSRI used for depression and OCD. While it may interact with other serotonergic drugs, it does not cause a disulfiram-like reaction. * **D. Acamprosate:** Used for maintaining abstinence in alcohol dependence by modulating glutamate/GABA systems. It does not affect alcohol metabolism and is safe to use even if a patient relapses. **High-Yield Clinical Pearls for NEET-PG:** * **Other drugs causing Disulfiram-like reactions:** Sulfonylureas (Chlorpropamide), Cephalosporins (Cefoperazone, Cefotetan), Griseofulvin, and Procarbazine. * **Disulfiram Mechanism:** Irreversible inhibition of Aldehyde Dehydrogenase. * **Management:** The reaction is usually self-limiting; management is supportive (IV fluids, antihistamines, and antiemetics). * **Contraindication:** Disulfiram should never be administered if the patient has consumed alcohol within the last 12 hours.

Q: Tranylcypromine, a Monoamine Oxidase Inhibitor, should be avoided with which of the following drug classes due to the risk of a dangerous drug interaction?

Tricyclic antidepressants. **Explanation:** The correct answer is **B. Tricyclic antidepressants (TCAs)**. **1. Why Tricyclic Antidepressants (TCAs) are the correct answer:** Tranylcypromine is a non-selective, irreversible Monoamine Oxidase Inhibitor (MAOI). MAOIs prevent the breakdown of neurotransmitters like serotonin and norepinephrine. When combined with TCAs (which inhibit the reuptake of these same amines), there is a synergistic increase in synaptic monoamine levels. This can precipitate **Serotonin Syndrome** (characterized by hyperthermia, rigidity, and autonomic instability) or a **Hypertensive Crisis**. Due to this dangerous interaction, a "washout period" of at least 14 days is mandatory when switching between these two classes. **2. Why the other options are incorrect:** * **A. Opioid analgesics:** While certain opioids (like Pethidine/Meperidine) are strictly contraindicated with MAOIs due to the risk of serotonin syndrome, not all opioids carry the same level of risk. However, the classic "dangerous interaction" taught in the context of antidepressant classes specifically highlights the TCA-MAOI combination. * **C. Benzodiazepines:** These drugs act on GABA receptors and do not significantly interact with the monoamine system. They are often safely co-prescribed with MAOIs to manage anxiety or insomnia. * **D. All of the above:** Incorrect because Benzodiazepines do not pose a "dangerous" interaction risk in this context. **High-Yield Clinical Pearls for NEET-PG:** * **Cheese Reaction:** MAOIs (like Tranylcypromine) interact with Tyramine-rich foods (aged cheese, wine), leading to a hypertensive crisis due to the displacement of norepinephrine. * **Pethidine Interaction:** Always remember the specific contraindication of Pethidine with MAOIs (Type I excitatory reaction). * **Drug of Choice:** Phentolamine (alpha-blocker) is the drug of choice for managing an MAOI-induced hypertensive crisis.

Q: Tacrolimus level is increased by all of the following drugs except?

Rifampicin. **Explanation:** The core concept behind this question is the **Cytochrome P450 (CYP3A4) enzyme system**. Tacrolimus, a potent calcineurin inhibitor used in organ transplantation, is primarily metabolized by the CYP3A4 isoenzyme in the liver and intestines. **1. Why Rifampicin is the correct answer:** Rifampicin is a classic, potent **enzyme inducer**. It increases the synthesis and activity of CYP3A4 enzymes. When co-administered with Tacrolimus, Rifampicin accelerates its metabolism, leading to **decreased** plasma levels of Tacrolimus. This can result in sub-therapeutic drug concentrations and an increased risk of graft rejection. **2. Why the other options are incorrect:** Options A, B, and C are all **enzyme inhibitors**: * **Erythromycin (Macrolide):** A well-known inhibitor of CYP3A4. * **Itraconazole (Azole antifungal):** One of the most potent inhibitors of the CYP3A4 system. * **Danazol (Androgen):** Also acts as a CYP3A4 inhibitor. These drugs decrease the metabolism of Tacrolimus, thereby **increasing** its serum concentration and increasing the risk of toxicity (e.g., nephrotoxicity, neurotoxicity). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Big" Inducers (Decrease Tacrolimus levels):** Rifampicin, Phenytoin, Carbamazepine, Phenobarbitone, and St. John’s Wort. * **The "Big" Inhibitors (Increase Tacrolimus levels):** Ketoconazole/Itraconazole, Erythromycin/Clarithromycin, Verapamil/Diltiazem, and **Grapefruit juice**. * **Monitoring:** Because Tacrolimus has a narrow therapeutic index, Therapeutic Drug Monitoring (TDM) is mandatory when starting or stopping any of the above medications. * **Cyclosporine** follows the same metabolic pathway and interaction profile as Tacrolimus.

Question 1

Which of the following is a P-Glycoprotein inhibitor?

Accepted Answer

Ketoconazole

Answer

Rifampicin

Answer

Phenytoin

Answer

Griseofulvin

Question 2

The anticoagulant effect of warfarin is increased by all of the following drugs, EXCEPT:

Accepted Answer

Phytonadione

Answer

Cimetidine

Answer

Amiodarone

Answer

Phenylbutazone

Question 3

Oral contraceptive pills' efficiency is reduced by the simultaneous use of which of the following medications?

Accepted Answer

Rifampicin and Carbamazepine

Answer

Rifampicin

Answer

Rifampicin and Tricyclic antidepressants

Answer

Carbamazepine and Propranolol

Question 4

Which of the following does not affect the metabolism of oral contraceptive pills?

Accepted Answer

Penicillin

Answer

Phenytoin

Answer

Griscofulvin

Answer

Primidone

Question 5

Disulfiram-like interaction with alcohol is seen with all of the following drugs except?

Accepted Answer

Satranidazole

Answer

Metronidazole

Answer

Cefoperazone

Answer

Griseofulvin

Question 6

Which of the following is not an inducer of CYP3A4?

Accepted Answer

Erythromycin

Answer

Barbiturates

Answer

Glucocorticoids

Answer

Phenytoin

Question 7

Sucralfate does not interfere with the absorption of which of the following drugs?

Accepted Answer

Phenoxymethylpenicillin

Answer

Ciprofloxacin

Answer

Phenytoin

Answer

Digoxin

Question 8

A disulfiram-like reaction is seen with which of the following medications?

Accepted Answer

Metronidazole

Answer

Benzodiazepine

Answer

Fluoxetine

Answer

Acamprosate

Question 9

Tranylcypromine, a Monoamine Oxidase Inhibitor, should be avoided with which of the following drug classes due to the risk of a dangerous drug interaction?

Accepted Answer

Tricyclic antidepressants

Answer

Opioid analgesics

Answer

Benzodiazepines

Answer

All of the above

Question 10

Tacrolimus level is increased by all of the following drugs except?

Accepted Answer

Rifampicin

Answer

Erythromycin

Answer

Itraconazole

Answer

Danazol

Drug Interactions — MCQs

Drug Interactions — MCQs

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