Drug Interactions Practice Questions

Q: Which of the following drugs can cause failure of oral contraceptive pills?

All of the above. The failure of oral contraceptive pills (OCPs) occurs primarily due to the induction of hepatic microsomal enzymes, specifically the **Cytochrome P450 (CYP3A4)** system. When these enzymes are induced, the metabolism of estrogen and progesterone components in OCPs is accelerated, leading to sub-therapeutic plasma levels and a loss of contraceptive efficacy. ### **Explanation of Options:** * **Rifampicin (Option A):** This is the most potent inducer of the CYP450 system. It significantly increases the hydroxylation of ethinylestradiol, making it the most common pharmacological cause of OCP failure. * **Nevirapine (Option B):** A Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI) used in HIV treatment, nevirapine is a known enzyme inducer that reduces the levels of hormonal contraceptives. * **Phenytoin (Option C):** This anti-epileptic drug (along with Carbamazepine and Phenobarbitone) induces hepatic enzymes, leading to rapid degradation of contraceptive steroids. Since all three drugs are potent enzyme inducers, **Option D (All of the above)** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Inducers":** Remember the mnemonic **"GPPRS Cell"** for enzyme inducers: **G**riseofulvin, **P**henytoin, **P**henobarbitone, **R**ifampicin, **S**t. John’s Wort, **C**arbamazepine. * **Management:** Patients on these medications should be advised to use an alternative or additional method of contraception (e.g., barrier methods or an IUD). * **Exception:** Unlike Rifampicin, the antibiotic **Penicillin** or **Tetracycline** may cause OCP failure by a different mechanism: disrupting the enterohepatic circulation of estrogens by killing gut flora. * **Pro-drug Note:** Enzyme inducers *decrease* the effect of most drugs but *increase* the toxicity of pro-drugs (e.g., Paracetamol) by increasing the formation of toxic metabolites.

Q: Which of the following is a Cytochrome P450 inhibitor?

Ketoconazole. **Explanation:** The correct answer is **Ketoconazole**. This question tests the fundamental pharmacological concept of drug-metabolizing enzyme modulation, specifically the Cytochrome P450 (CYP450) system. **1. Why Ketoconazole is correct:** Ketoconazole is a potent **enzyme inhibitor** [1], [2], [3]. It binds to the iron atom of the heme group in CYP450 enzymes (specifically CYP3A4), preventing the oxidation of other drugs [1]. This leads to decreased metabolism and increased plasma concentrations of co-administered drugs (e.g., Warfarin, Statins), potentially leading to toxicity [2]. **2. Why the other options are incorrect:** * **Rifampicin:** This is a classic, potent **enzyme inducer** [2]. It increases the synthesis of CYP450 enzymes, leading to faster metabolism and reduced efficacy of co-administered drugs (e.g., Oral Contraceptive Pills, leading to contraceptive failure). * **Phenytoin:** This is also a well-known **enzyme inducer**. Like Rifampicin, it accelerates the clearance of other drugs metabolized by the liver. **3. High-Yield Clinical Pearls for NEET-PG:** To remember these for the exam, use these popular mnemonics: * **Enzyme Inhibitors (VITAMINS K):** **V**alproate, **I**soniazid, **T**rimethoprim, **A**miodarone, **M**acrolides (except Azithromycin), **I**ndinavir, **N**etilmicin (and other Azoles like **Ketoconazole**), **S**ulfonamides, **C**imetidine, **G**rapefruit juice. * **Enzyme Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. **Key Fact:** Ketoconazole is also known to inhibit steroid synthesis (adrenal and gonadal) [1], [3], which is why it can cause side effects like gynecomastia in males.

Q: Which of the following drugs, if given with terfenadine, can cause ventricular arrhythmias?

Ketoconazole. **Explanation:** The correct answer is **Ketoconazole**. **Mechanism of Interaction:** Terfenadine is a second-generation H1-antihistamine that acts as a prodrug. It is normally metabolized by the hepatic cytochrome P450 enzyme **CYP3A4** into its active, non-toxic metabolite, fexofenadine. **Ketoconazole** is a potent inhibitor of CYP3A4. When co-administered, ketoconazole inhibits the metabolism of terfenadine, leading to toxic systemic accumulation of the parent drug. High levels of terfenadine block the delayed rectifier potassium channels ($I_{Kr}$) in the heart, causing **prolongation of the QT interval**, which can progress to a life-threatening polymorphic ventricular tachycardia known as **Torsades de Pointes (TdP)**. **Analysis of Incorrect Options:** * **A. Griseofulvin:** This is an antifungal that acts as a CYP450 **inducer**, not an inhibitor. It would decrease the levels of drugs metabolized by these enzymes rather than causing toxicity. * **C. Ampicillin:** This is a penicillin antibiotic that does not significantly inhibit the CYP3A4 system and has no known cardiotoxic interactions with antihistamines. * **D. Sparfloxacin:** While fluoroquinolones like sparfloxacin can independently cause QT prolongation, the classic, high-yield drug interaction specifically linked to the withdrawal of terfenadine from the market involves CYP3A4 inhibitors like Ketoconazole or Erythromycin. **High-Yield Facts for NEET-PG:** * **"Dangerous Duos":** Terfenadine or Astemizole + Erythromycin/Clarithromycin or Ketoconazole/Itraconazole. * **Safe Alternative:** **Fexofenadine** is the active metabolite of terfenadine; it does not block potassium channels and is safe from this specific interaction. * Other drugs causing QT prolongation: Class IA and III antiarrhythmics, Tricyclic antidepressants, and Haloperidol.

Q: A patient with HIV disease is on Highly Active Anti Retroviral Therapy (HAART). The patient develops tuberculosis. Which one of the following anti-tubercular drugs interferes with HAART?

Rifampicin. **Explanation:** The correct answer is **Rifampicin**. **Why Rifampicin is the correct answer:** Rifampicin is a potent **inducer of the Cytochrome P450 (CYP450) enzyme system**, specifically the CYP3A4 isoenzyme. Most Protease Inhibitors (PIs) and Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs) used in HAART are substrates of this same enzyme system. By inducing these enzymes, Rifampicin significantly increases the metabolism of antiretroviral drugs, leading to sub-therapeutic plasma levels, treatment failure, and the potential development of drug-resistant HIV strains. **Why the other options are incorrect:** * **Isoniazid (INH):** While INH can inhibit certain CYP enzymes, it does not have the profound inducing effect on HAART metabolism that Rifampicin does. Its primary interaction concern is peripheral neuropathy, which can be additive if the patient is on Stavudine (d4T). * **Ethambutol:** This drug is primarily excreted renally and does not significantly interfere with the hepatic microsomal enzyme system. * **Pyrazinamide:** Like Ethambutol, Pyrazinamide does not have significant induction or inhibition effects on the CYP450 system and does not interfere with the efficacy of HAART. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rifabutin" Alternative:** In HIV patients on HAART, **Rifabutin** is often substituted for Rifampicin because it is a much less potent inducer of CYP3A4, making it safer to co-administer with Protease Inhibitors. * **Drug of Choice:** For TB in HIV patients, the standard "RHZE" regimen is used, but the HAART regimen may need adjustment (e.g., using Efavirenz-based regimens or Integrase Inhibitors like Dolutegravir at a higher dose). * **IRIS:** Be aware of **Immune Reconstitution Inflammatory Syndrome (IRIS)**, which can occur when starting HAART in a TB patient due to a sudden recovery of the immune response against TB antigens.

Q: Which drug causes synergism with neuromuscular blockers, leading to blockage?

Tobramycin. **Explanation:** The correct answer is **Tobramycin**. This interaction is a high-yield concept in anesthesia and pharmacology involving the potentiation of neuromuscular blockade. **1. Why Tobramycin is Correct:** Tobramycin is an **Aminoglycoside**. Aminoglycosides interfere with neuromuscular transmission by two primary mechanisms: * **Presynaptic:** They inhibit the release of Acetylcholine (ACh) from the motor nerve terminal by competing with Calcium ions at voltage-gated P/Q-type calcium channels. * **Postsynaptic:** They reduce the sensitivity of the nicotinic receptors (Nm) to ACh. When administered with neuromuscular blockers (like Vecuronium or Succinylcholine), aminoglycosides cause **synergistic blockade**, potentially leading to prolonged apnea or respiratory paralysis. This effect can be partially reversed by Calcium gluconate or Neostigmine. **2. Why Other Options are Incorrect:** * **Vancomycin (A):** While Vancomycin can cause "Red Man Syndrome" due to histamine release, it does not typically interfere with the neuromuscular junction in the same synergistic manner as aminoglycosides. * **Erythromycin (C):** As a Macrolide, it is primarily known for inhibiting CYP3A4 enzymes and causing QT prolongation, but it does not significantly potentiate neuromuscular blockers. * **Amoxycillin (D):** Penicillins do not have any documented inhibitory effect on the neuromuscular junction. **Clinical Pearls for NEET-PG:** * **Mnemonic for drugs causing NMJ blockade:** "A-L-Q" (**A**minoglycosides, **L**ignocaine/Local Anesthetics, **Q**uinidine/Quinine). * **Rank of Potency:** Among aminoglycosides, Neomycin > Streptomycin > Amikacin > Gentamicin in terms of NMJ blocking potential. * **Contraindication:** Aminoglycosides are strictly contraindicated in patients with **Myasthenia Gravis** as they can precipitate a myasthenic crisis.

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

Cimetidine. **Explanation:** The correct answer is **Cimetidine**. The interaction between drugs and Warfarin is a high-yield topic in NEET-PG, primarily revolving around the **Cytochrome P450 (CYP450) enzyme system**. **1. Why Cimetidine is Correct:** Warfarin is metabolized by hepatic enzymes, specifically **CYP2C9**. Cimetidine is a potent **enzyme inhibitor**. By inhibiting these enzymes, Cimetidine reduces the metabolism of Warfarin, leading to increased plasma levels of the drug. This enhances its anticoagulant activity, significantly increasing the risk of bleeding (elevated INR). **2. Why the Other Options are Incorrect:** * **Phenytoin & Griseofulvin:** These are classic **enzyme inducers**. They increase the synthesis of CYP450 enzymes, which accelerates the metabolism of Warfarin. This decreases Warfarin's plasma concentration and reduces its anticoagulant effect (increased risk of thrombosis). * **Oral Contraceptive Pills (OCP):** OCPs increase the synthesis of clotting factors (II, VII, IX, and X) in the liver. This physiological effect antagonizes the action of Warfarin, effectively decreasing its efficacy. **Clinical Pearls for NEET-PG:** * **S-Warfarin** is more potent than R-Warfarin and is metabolized by **CYP2C9**. * **Mnemonic for Enzyme Inhibitors (Increase Warfarin activity):** **VITAMINS K** – **V**erapamil, **I**soniazid, **T**rimethoprim, **A**miodarone, **M**etronidazole, **I**ndinavir, **N**eomycin, **S**ulfonamides, **K**etoconazole (and Cimetidine/Erythromycin). * **Mnemonic for Enzyme Inducers (Decrease Warfarin activity):** **G P R S Cell Phone** – **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: Patients should be cautioned not to consume alcohol when given a prescription for any of the following drugs EXCEPT:

Cefixime. The core concept behind this question is the **Disulfiram-like reaction**. This occurs when certain drugs inhibit the enzyme **aldehyde dehydrogenase**, leading to the accumulation of acetaldehyde in the blood after alcohol consumption. This results in distressing symptoms like flushing, palpitations, tachycardia, nausea, and hypotension. **Why Cefixime is the Correct Answer:** Cefixime is a third-generation oral cephalosporin that **does not** contain the specific chemical moiety responsible for inhibiting aldehyde dehydrogenase. Therefore, it does not cause a disulfiram-like reaction and is safe to use with alcohol. **Analysis of Incorrect Options:** * **Cefoperazone:** This is a third-generation injectable cephalosporin that contains a **Methylthiotetrazole (MTT) side chain**. This specific side chain is responsible for inhibiting aldehyde dehydrogenase and causing the reaction. * **Chlorpropamide:** A first-generation sulfonylurea used in diabetes. It is classic for causing a disulfiram-like flush, a common high-yield fact in endocrine pharmacology. * **Metronidazole:** An antiprotozoal/antibiotic that is perhaps the most well-known trigger for this reaction. Patients are routinely cautioned to avoid alcohol for at least 48–72 hours after the last dose. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cephalosporins with MTT side chains:** Cefoperazone, Cefotetan, and Cefamandole (Mnemonic: **"MAN operates the TETan"**). 2. **Other drugs causing Disulfiram-like reactions:** Procarbazine (anticancer), Griseofulvin (antifungal), and Tinidazole. 3. **Mechanism:** Inhibition of **Aldehyde Dehydrogenase** (not Alcohol Dehydrogenase). 4. **Cefixime Safety:** It is the drug of choice for uncomplicated gonorrhea and is generally free from the bleeding risks (hypoprothrombinemia) associated with MTT-containing cephalosporins.

Q: Concomitant use of statins with which of the following drugs can cause rhabdomyolysis?

Rifampin. **Explanation:** The primary mechanism behind statin-induced rhabdomyolysis is the inhibition of the **CYP3A4 enzyme**, which leads to increased plasma concentrations of statins (especially Simvastatin, Lovastatin, and Atorvastatin). **Why Rifampin is the Correct Answer (in the context of this specific question):** While Rifampin is a potent **CYP450 inducer** (which usually decreases drug levels), it interacts with statins through the inhibition of **OATP1B1 (Organic Anion Transporting Polypeptide)** hepatic uptake transporters. Acute administration or specific competition at this transporter can paradoxically increase systemic statin exposure, leading to muscle toxicity. *Note: In many clinical scenarios, Rifampin decreases statin levels over time due to induction, but it remains a high-yield "exception" drug in competitive exams regarding transporter-mediated interactions.* **Analysis of Other Options:** * **Macrolides (e.g., Erythromycin, Clarithromycin):** These are potent **CYP3A4 inhibitors**. They significantly increase statin levels and are a very common cause of rhabdomyolysis. * **Azoles (e.g., Ketoconazole, Itraconazole):** These are also strong **CYP3A4 inhibitors** that decrease statin metabolism, raising the risk of myopathy. * **Fibrates (especially Gemfibrozil):** These increase the risk of rhabdomyolysis via a pharmacodynamic synergy and by inhibiting the **glucuronidation** of statins, which is essential for their excretion. **NEET-PG High-Yield Pearls:** 1. **Gemfibrozil + Statin** is the most dangerous combination for rhabdomyolysis (Fenofibrate is safer). 2. **Pravastatin and Rosuvastatin** are not primarily metabolized by CYP3A4, making them safer choices when using CYP inhibitors. 3. **Clinical Sign:** Look for elevated **Creatine Kinase (CK)** levels and myoglobinuria (tea-colored urine) in clinical vignettes.

Q: In theophylline metabolism, drug interactions occur with all of the following except:

Tetracyclines. **Explanation:** Theophylline has a **narrow therapeutic index** (10-20 µg/ml) and is primarily metabolized by the hepatic **Cytochrome P450 (CYP1A2 and CYP3A4)** enzyme system. Consequently, its serum levels are highly sensitive to drugs that induce or inhibit these enzymes. **1. Why Tetracyclines is the correct answer:** Tetracyclines (like Doxycycline or Oxytetracycline) do not significantly inhibit or induce hepatic microsomal enzymes. Therefore, they do not alter the metabolism of theophylline. While some Macrolides (like Erythromycin) and Fluoroquinolones (like Ciprofloxacin) are notorious for increasing theophylline levels, **Tetracyclines are considered safe** in this regard. **2. Analysis of Incorrect Options:** * **Cimetidine:** A potent **enzyme inhibitor**. It inhibits CYP1A2 and CYP3A4, leading to decreased clearance of theophylline, which can result in theophylline toxicity (nausea, palpitations, seizures). * **Phenobarbitone:** A classic **enzyme inducer**. It increases the synthesis of microsomal enzymes, accelerating theophylline metabolism and potentially leading to sub-therapeutic levels and poor asthma control. * **Rifampicin:** A powerful **broad-spectrum enzyme inducer**. Similar to Phenobarbitone, it enhances the clearance of theophylline, necessitating a dosage increase. **High-Yield Clinical Pearls for NEET-PG:** * **Theophylline Toxicity:** Manifests as persistent vomiting, cardiac arrhythmias, and intractable seizures. * **Other Inhibitors (Increase levels):** Erythromycin, Ciprofloxacin, Oral Contraceptives, and Allopurinol. * **Other Inducers (Decrease levels):** Phenytoin, Carbamazepine, and **Smoking** (Polycyclic hydrocarbons induce CYP1A2). * **Mechanism:** Theophylline works by inhibiting Phosphodiesterase (PDE) and antagonizing Adenosine receptors.

Question 1

Which of the following drugs can cause failure of oral contraceptive pills?

Accepted Answer

All of the above

Answer

Rifampicin

Answer

Nevirapine

Answer

Phenytoin

Question 2

Which of the following is a Cytochrome P450 inhibitor?

Accepted Answer

Ketoconazole

Answer

Rifampicin

Answer

Phenytoin

Answer

None of the above

Question 3

Which of the following drugs, if given with terfenadine, can cause ventricular arrhythmias?

Accepted Answer

Ketoconazole

Answer

Griseofulvin

Answer

Ampicillin

Answer

Sparfloxacin

Question 4

A patient with HIV disease is on Highly Active Anti Retroviral Therapy (HAART). The patient develops tuberculosis. Which one of the following anti-tubercular drugs interferes with HAART?

Accepted Answer

Rifampicin

Answer

Isoniazid

Answer

Ethambutol

Answer

Pyrazinamide

Question 5

Which drug causes synergism with neuromuscular blockers, leading to blockage?

Accepted Answer

Tobramycin

Answer

Vancomycin

Answer

Erythromycin

Answer

Amoxycillin

Question 6

Which of the following drugs increases the activity of warfarin?

Accepted Answer

Cimetidine

Answer

Oral contraceptive pills (OCP)

Answer

Griseofulvin

Answer

Phenytoin

Question 7

A 30-year-old mother of 2 children weighing 60 kg was taking combined oral contraceptive pills containing levonorgestrel 0.15 mg + ethinylestradiol 30 μg per day cyclically. She developed fever with cough and was diagnosed with pulmonary tuberculosis. She was treated with isoniazid (300 mg) + rifampin (600 mg) + pyrazinamide (1.5 g) + ethambutol (1.0 g) daily for 2 months, followed by isoniazid (600 mg) + rifampin (600 mg) thrice weekly. In the 3rd month, she failed to have the usual withdrawal bleeding during the gap period of her contraceptive cycle. After 10 days, her urinary pregnancy test was found to be positive. What could be the reason for the failure of the oral contraceptive?

Accepted Answer

Rifampicin is an enzyme inducer and induced the metabolism of contraceptives.

Answer

Ethambutol is an enzyme inducer and induced the metabolism of contraceptives.

Answer

Both rifampicin and ethambutol are enzyme inducers and induced the metabolism of contraceptives.

Answer

None of the above.

Question 8

Patients should be cautioned not to consume alcohol when given a prescription for any of the following drugs EXCEPT:

Accepted Answer

Cefixime

Answer

Cefoperazone

Answer

Chlorpropamide

Answer

Metronidazole

Question 9

Concomitant use of statins with which of the following drugs can cause rhabdomyolysis?

Accepted Answer

Rifampin

Answer

Macrolide antibiotic

Answer

Azoles (antifungal)

Answer

Fibrates

Question 10

In theophylline metabolism, drug interactions occur with all of the following except:

Accepted Answer

Tetracyclines

Answer

Cimetidine

Answer

Phenobarbitone

Answer

Rifampicin

Drug Interactions — MCQs

Drug Interactions — MCQs

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