Drug Interactions Practice Questions

Q: A patient taking both ketoconazole and terfenadine is prone to which of the following?

Cardiac arrhythmia. ### Explanation The correct answer is **Cardiac Arrhythmia (Option A)**. **Mechanism of Interaction:** This is a classic example of a **pharmacokinetic drug interaction** involving the inhibition of cytochrome P450 enzymes. 1. **Terfenadine** is a non-sedating antihistamine that acts as a pro-drug. It is normally metabolized by the isoenzyme **CYP3A4** into its active metabolite, fexofenadine. 2. **Ketoconazole** is a potent **CYP3A4 inhibitor**. 3. When taken together, ketoconazole inhibits the metabolism of terfenadine, leading to toxic accumulation of the parent drug in the plasma. 4. High levels of terfenadine block the **delayed rectifier potassium channels (IKr)** in the heart, prolonging the action potential duration. This manifests as **QT interval prolongation** on an ECG, which can trigger a life-threatening polymorphic ventricular tachycardia known as **Torsades de Pointes**. **Analysis of Incorrect Options:** * **B. Toxicity of ketoconazole:** The interaction is unidirectional in this context; terfenadine does not inhibit the metabolism of ketoconazole. * **C. Congestive cardiac failure:** While arrhythmias can worsen existing heart failure, the primary and immediate risk of this specific interaction is an electrical conduction abnormality (arrhythmia), not structural pump failure. **NEET-PG High-Yield Pearls:** * **The "Terfenadine Rule":** Due to this fatal interaction, terfenadine (and astemizole) have been largely replaced by **fexofenadine**, which does not cause QT prolongation even at high concentrations. * **Other CYP3A4 Inhibitors:** Be wary of "VITamin K" (Verapamil, Itraconazole/Ketoconazole, Troleandomycin, Amiodarone, Macrolides like Erythromycin/Clarithromycin, and Grapefruit juice) when prescribed with QT-prolonging drugs. * **Other drugs causing Torsades:** Class IA and III antiarrhythmics, Tricyclic antidepressants, and Fluoroquinolones.

Q: Which one of the following drugs can reduce the efficacy of a low-dose oral contraceptive pill?

Rifampicin. **Explanation:** The correct answer is **Rifampicin**. **1. Why Rifampicin is correct:** Rifampicin is a potent **microsomal enzyme inducer** (specifically inducing CYP3A4). Oral contraceptive pills (OCPs) contain estrogen (Ethinylestradiol) and progestogens, which are metabolized by the cytochrome P450 system in the liver. By inducing these enzymes, Rifampicin significantly accelerates the metabolism of these hormones, leading to decreased plasma concentrations and a high risk of **contraceptive failure**. This is a classic example of a pharmacokinetic drug interaction. **2. Why other options are incorrect:** * **Penicillin, Tetracycline, and Ampicillin:** Historically, it was believed that broad-spectrum antibiotics reduced OCP efficacy by altering gut flora and interrupting the **enterohepatic circulation** of estrogen. However, clinical studies have shown that these antibiotics do not significantly lower hormone levels in most women. Current clinical guidelines (like those from the CDC and Faculty of Sexual and Reproductive Healthcare) state that non-enzyme-inducing antibiotics do not require extra contraceptive precautions, unlike Rifampicin. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Rifa" Rule:** Rifampicin and Rifabutin are the only antibiotics proven to consistently reduce OCP efficacy. * **Other Enzyme Inducers:** Remember the mnemonic **"GPPRS"** (Griseofulvin, Phenytoin, Phenobarbitone, Rifampicin, St. John's Wort) and **Carbamazepine**. All these can cause OCP failure. * **Clinical Advice:** Patients on Rifampicin should be advised to use an alternative method of contraception (e.g., condoms or an IUD) during the course and for 28 days after stopping the drug. * **Exception:** Rifampicin does *not* affect the efficacy of the Progestogen-only injectable (DMPA) or the Copper-T IUD.

Q: A 27-year-old man with epilepsy presents with complaints of persistent lethargy and occasional feelings of intoxication. He does not consume alcohol and is currently taking phenobarbital for the long-term management of tonic-clonic seizures. He was recently started on a medication for the treatment of gastroesophageal reflux disease. Which of the following agents was he most likely prescribed?

Cimetidine. **Explanation:** The patient is experiencing symptoms of **phenobarbital toxicity** (lethargy, ataxia, and "intoxication" feelings) due to a drug-drug interaction. Phenobarbital is a narrow therapeutic index drug metabolized by the hepatic cytochrome P450 (CYP450) enzyme system. **1. Why Cimetidine is Correct:** Cimetidine is a potent **non-specific inhibitor of the CYP450 enzyme system** (specifically CYP1A2, 2C9, 2D6, and 3A4). When added to a stable regimen of phenobarbital, cimetidine inhibits its metabolism, leading to increased plasma levels of the anticonvulsant and subsequent toxicity. **2. Why the Other Options are Incorrect:** * **Famotidine and Ranitidine:** These are also $H_2$ receptor antagonists used for GERD. However, unlike cimetidine, they have **negligible effects** on the CYP450 system and do not typically cause significant drug interactions with antiepileptics. * **Lansoprazole:** While some Proton Pump Inhibitors (PPIs) like Omeprazole can inhibit certain CYP isoforms (e.g., CYP2C19), Lansoprazole has a much lower potential for clinically significant interactions compared to the classic enzyme inhibitor, Cimetidine. **High-Yield Clinical Pearls for NEET-PG:** * **Cimetidine Side Effects:** Apart from enzyme inhibition, it has **anti-androgenic effects** (gynecomastia, galactorrhea, and decreased libido) because it displaces DHT from its receptors and inhibits the metabolism of estradiol. * **Mnemonic for Enzyme Inhibitors:** "**SICKFACES.COM**" (Sodium valproate, Isoniazid, Cimetidine, Ketoconazole, Fluconazole, Alcohol (acute), Chloramphenicol, Erythromycin, Sulfonamides, Ciprofloxacin, Omeprazole, Metronidazole). * **Phenobarbital:** It is a potent enzyme **inducer**, but in this scenario, it is the *substrate* being affected by the *inhibitor* (Cimetidine).

Q: In theophylline metabolism, drug interactions occur with all EXCEPT:

Tetracyclines. **Explanation:** Theophylline has a narrow therapeutic index and is primarily metabolized by the hepatic **Cytochrome P450 (CYP1A2 and CYP3A4)** enzyme system. Therefore, any drug that induces or inhibits these enzymes will significantly alter theophylline serum levels. **1. Why Tetracyclines is the Correct Answer:** Tetracyclines (like Doxycycline or Oxytetracycline) do not significantly inhibit or induce hepatic microsomal enzymes. Consequently, they do not interfere with theophylline clearance. It is a common "distractor" in exams because **Macrolides** (like Erythromycin and Clarithromycin) are potent inhibitors that *do* interact with theophylline, but Tetracyclines do not. **2. Analysis of Incorrect Options:** * **Cimetidine:** A well-known **Enzyme Inhibitor**. It reduces the metabolism of theophylline, leading to increased serum levels and potential toxicity (tachycardia, seizures). * **Phenobarbitone:** A potent **Enzyme Inducer**. It increases the synthesis of CYP enzymes, leading to faster metabolism of theophylline and decreased therapeutic efficacy. * **Rifampicin:** Another powerful **Enzyme Inducer**. Like Phenobarbitone, it enhances theophylline clearance, necessitating a dose increase to maintain bronchodilation. **High-Yield Clinical Pearls for NEET-PG:** * **Ciprofloxacin (Fluoroquinolone):** Frequently tested as a potent inhibitor of theophylline metabolism. * **Smoking:** Tobacco smoke induces CYP1A2, which *increases* theophylline clearance (smokers need higher doses). * **Theophylline Toxicity:** Early signs include GI upset and restlessness; severe toxicity leads to **cardiac arrhythmias** and **intractable seizures**. * **Mnemonic for Inducers:** "GPRS Cell Phone" (Griseofulvin, Phenytoin, Rifampicin, Smoking, Carbamazepine, Phenobarbitone).

Q: A patient on lithium therapy was found to be hypertensive. Which of the following antihypertensive drugs is contraindicated in a patient on lithium therapy to prevent toxicity?

Diuretics. **Explanation:** **1. Why Diuretics are the Correct Answer:** The primary mechanism behind lithium toxicity with diuretics (specifically **Thiazides** and **Loop diuretics**) involves the renal handling of sodium. Lithium is an alkali metal that the kidneys treat similarly to sodium. When diuretics cause sodium depletion, the proximal tubule compensates by increasing the reabsorption of sodium to maintain homeostasis. Because the renal tubules cannot distinguish between sodium and lithium ions, **lithium reabsorption is also increased**, leading to a significant rise in serum lithium levels and subsequent toxicity. **2. Analysis of Incorrect Options:** * **Clonidine (A):** This centrally acting alpha-2 agonist does not significantly interfere with the renal clearance of lithium. * **Beta blockers (B):** These are often used intentionally alongside lithium to treat lithium-induced fine tremors (Propranolol is the drug of choice). They do not increase lithium levels. * **Calcium channel blockers (C):** While some CCBs (like Verapamil) may rarely cause neurotoxicity when combined with lithium, they do not typically cause a rise in serum lithium levels through renal mechanisms. They are not strictly contraindicated compared to the high risk posed by diuretics. **3. NEET-PG High-Yield Pearls:** * **The "Big Three" Interactions:** Drugs that increase lithium levels are **Diuretics** (Thiazides > Loop), **NSAIDs** (except Aspirin/Sulindac), and **ACE inhibitors/ARBs**. * **Exception:** **Acetazolamide** and **Theophylline** actually *increase* lithium excretion, potentially lowering its therapeutic effect. * **Clinical Sign:** Lithium has a narrow therapeutic index (0.6–1.2 mEq/L). Toxicity presents with coarse tremors, ataxia, vomiting, and polyuria. * **Safe Alternative:** If a patient on lithium requires an antihypertensive, **Amlodipine** (CCB) is generally considered a safer choice.

Q: Which of the following drugs reduces the efficacy of combined oral contraceptive pills?

Griseofulvin. **Explanation:** The correct answer is **Griseofulvin**. **Mechanism of Interaction:** Combined Oral Contraceptive Pills (COCPs) are primarily metabolized in the liver by the **Cytochrome P450 (CYP450)** enzyme system, specifically the **CYP3A4** isoenzyme [1]. **Griseofulvin** is a potent hepatic enzyme inducer. When co-administered, it increases the rate of metabolism of estrogen and progesterone components of the pill. This leads to decreased plasma concentrations of these hormones, potentially falling below the therapeutic threshold required to suppress ovulation, thereby resulting in **contraceptive failure** [2]. **Analysis of Incorrect Options:** * **Diazepam:** It is a benzodiazepine that is metabolized by the liver but does not significantly induce or inhibit enzymes in a way that affects COCP efficacy. In fact, COCPs may actually inhibit the metabolism of diazepam, potentially increasing its sedation. * **Aspirin & Paracetamol:** These are non-opioid analgesics. They do not induce hepatic enzymes. While high doses of Vitamin C or Paracetamol can occasionally compete for sulfate conjugation (potentially increasing estrogen levels slightly), they do not reduce the efficacy of the pill. **High-Yield Clinical Pearls for NEET-PG:** * **Other Enzyme Inducers:** Remember the mnemonic **"GP Cell Phone"** or **"GPRS"** for drugs that reduce COCP efficacy: **G**riseofulvin, **P**henytoin, **R**ifampicin (the most potent), and **S**t. John’s Wort (herbal) [1], along with Carbamazepine [2] and Phenobarbitone. * **Rifampicin Alert:** Rifampicin is a classic "favorite" for NEET-PG; it is the only antibiotic with a proven, clinically significant reduction in COCP levels due to enzyme induction. * **Clinical Advice:** Patients on enzyme-inducers should be advised to use an alternative method of contraception (e.g., IUCD) or a barrier method during and for 28 days after stopping the inducer.

Q: Which drug should not be given with Apomorphine?

Ondansetron. Apomorphine is a potent dopamine agonist used as "rescue therapy" for "off" episodes in Parkinson’s disease [1]. A major side effect of apomorphine is severe nausea and vomiting [2]. However, **5-HT3 antagonists (like Ondansetron)** are strictly contraindicated for managing this emesis. When used together, there is a risk of **profound hypotension and loss of consciousness**. The exact mechanism is not fully understood but is clinically significant enough to warrant a "Black Box Warning." To manage apomorphine-induced vomiting, the drug of choice is **Trimethobenzamide**. **2. Analysis of Incorrect Options:** * **A. Dopamine Agonists:** Apomorphine itself is a non-ergoline dopamine agonist [1]. While adding other agonists (like Pramipexole) requires dose monitoring for additive dopaminergic side effects (dyskinesia, hallucinations) [3], it is not a contraindication; in fact, apomorphine is often used as an adjunct to existing dopaminergic therapy. * **B. Spironolactone:** This is a potassium-sparing diuretic. There is no significant pharmacological interaction between spironolactone and apomorphine. * **C. Aspirin:** Aspirin is an NSAID/antiplatelet agent. It does not interfere with the dopaminergic pathways or the hemodynamic stability associated with apomorphine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice for Apomorphine Emesis:** Trimethobenzamide (started 3 days prior to apomorphine). * **Avoid Domperidone/Metoclopramide:** While Domperidone is often used in Parkinson's, 5-HT3 antagonists (Ondansetron) are the specific "Never-Give" combination with Apomorphine due to the syncope/hypotension risk. * **Apomorphine Route:** Administered **subcutaneously**, not orally (due to high first-pass metabolism) [1]. * **Other Uses:** Historically used as an emetic in toxicology (now obsolete).

Q: A patient stabilized on warfarin for DVT is given phenobarbitone. Which of the following is the likely interaction between the two drugs?

Phenobarbitone increases the metabolism of warfarin. **Explanation:** The interaction between warfarin and phenobarbitone is a classic example of **pharmacokinetic drug antagonism** mediated by **enzyme induction** [1], [2]. **Why Option B is Correct:** Phenobarbitone is a potent inducer of the hepatic microsomal enzyme system, specifically the **Cytochrome P450 (CYP) enzymes** (primarily CYP2C9, which metabolizes warfarin) [1]. By inducing these enzymes, phenobarbitone increases the rate of warfarin metabolism [1]. This leads to decreased plasma concentrations of warfarin, a shorter half-life, and a reduced anticoagulant effect [1]. Clinically, this necessitates an increase in the warfarin dose to maintain a therapeutic International Normalized Ratio (INR) [1]. **Why Other Options are Incorrect:** * **Option A:** Phenobarbitone does not significantly affect the gastrointestinal absorption of warfarin. Warfarin is already rapidly and almost completely absorbed [3]. * **Option C:** While many drugs (like sulfonamides or NSAIDs) interact with warfarin via protein displacement, phenobarbitone does not [2]. Displacement would lead to an *increased* free fraction of warfarin and a higher risk of bleeding, which is the opposite of what occurs with phenobarbitone [1]. **High-Yield Clinical Pearls for NEET-PG:** 1. **G-P-R-S-Cell Phone:** A common mnemonic for **Enzyme Inducers**: **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, and **P**henobarbitone. 2. **Clinical Consequence:** If phenobarbitone is suddenly stopped in a patient on warfarin, the enzyme induction wears off, warfarin metabolism slows down, and the patient faces a high risk of **life-threatening hemorrhage** [1]. 3. **Warfarin Metabolism:** Warfarin is primarily metabolized by **CYP2C9** [2], [3]. Genetic polymorphisms in this enzyme can also affect a patient's dose requirements [3].

Q: Which drug does not result in theophylline toxicity?

Amoxicillin. **Explanation:** Theophylline is a methylxanthine bronchodilator with a **narrow therapeutic index** (10–20 µg/ml). It is primarily metabolized in the liver by the **Cytochrome P450 (CYP1A2 and CYP3A4)** enzyme systems. Any drug that inhibits these enzymes will decrease the clearance of theophylline, leading to toxic levels in the blood. * **Why Amoxicillin is correct:** Amoxicillin is a penicillin-group antibiotic that does **not** inhibit hepatic microsomal enzymes. It has no significant effect on theophylline metabolism, making it safe to co-administer without the risk of toxicity. * **Why other options are incorrect:** * **Ciprofloxacin:** A potent inhibitor of **CYP1A2**. It significantly reduces theophylline clearance, often requiring a 30-50% dose reduction of theophylline. * **Erythromycin:** A macrolide that inhibits **CYP3A4**. It is a classic cause of drug-induced theophylline toxicity. * **Cimetidine:** A well-known non-specific **enzyme inhibitor** that interferes with multiple CYP450 pathways, leading to increased serum levels of theophylline. **High-Yield Clinical Pearls for NEET-PG:** 1. **Theophylline Toxicity Symptoms:** Characterized by persistent vomiting, cardiac arrhythmias, and generalized seizures (refractory to diazepam). 2. **Other Inhibitors (Increase Toxicity):** Clarithromycin, Allopurinol, Propranolol, and Oral Contraceptive Pills. 3. **Enzyme Inducers (Decrease Efficacy):** Phenytoin, Rifampicin, and **Smoking** (induces CYP1A2) decrease theophylline levels, necessitating higher doses. 4. **Mnemonic:** Remember **"VITAMINS"** for enzyme inhibitors: **V**alproate, **I**soniazid, **T**rimethoprim, **A**miodarone, **M**acrolides (except Azithromycin), **I**traconazole, **N**euroleptics, **S**ulfonamides/Cimetidine.

Question 1

A patient taking both ketoconazole and terfenadine is prone to which of the following?

Accepted Answer

Cardiac arrhythmia

Answer

Toxicity of ketoconazole

Answer

Congestive cardiac failure

Answer

All of the above

Question 2

Which one of the following drugs can reduce the efficacy of a low-dose oral contraceptive pill?

Accepted Answer

Rifampicin

Answer

Penicillin

Answer

Tetracycline

Answer

Ampicillin

Question 3

A 27-year-old man with epilepsy presents with complaints of persistent lethargy and occasional feelings of intoxication. He does not consume alcohol and is currently taking phenobarbital for the long-term management of tonic-clonic seizures. He was recently started on a medication for the treatment of gastroesophageal reflux disease. Which of the following agents was he most likely prescribed?

Accepted Answer

Cimetidine

Answer

Famotidine

Answer

Lansoprazole

Answer

Ranitidine

Question 4

In theophylline metabolism, drug interactions occur with all EXCEPT:

Accepted Answer

Tetracyclines

Answer

Cimetidine

Answer

Phenobarbitone

Answer

Rifampicin

Question 5

A patient on lithium therapy was found to be hypertensive. Which of the following antihypertensive drugs is contraindicated in a patient on lithium therapy to prevent toxicity?

Accepted Answer

Diuretics

Answer

Clonidine

Answer

Beta blockers

Answer

Calcium channel blockers

Question 6

Which of the following drugs reduces the efficacy of combined oral contraceptive pills?

Accepted Answer

Griseofulvin

Answer

Diazepam

Answer

Aspirin

Answer

Paracetamol

Question 7

A 58-year-old woman with a history of alcohol abuse, coronary artery disease, and atrial fibrillation is on metoprolol, lisinopril, simvastatin, and warfarin. She develops urinary urgency and frequency and is treated with oxycodone and ciprofloxacin. Three days later, she develops a headache, dizziness, vomiting, and difficulty walking. Neurological examination reveals normal strength, sensation (including vibratory sensation), and reflexes. She has an uncoordinated, unsteady gait. Testing of coordination in the upper extremities shows past pointing and poor rapid alternating movements with her right upper extremity. In the lower extremities, her heel-shin testing also reveals poor coordination on the right. Her INR is 6.5 (normal <1; therapeutic for warfarin is 2.0-3.0). What is the most likely cause of her neurologic findings?

Accepted Answer

Right cerebellar hemorrhage

Answer

Multiple small infarcts in the basal ganglia

Answer

Cerebellar degeneration due to chronic alcohol abuse

Answer

Posterior column degeneration as a result of vitamin deficiency

Question 8

Which drug should not be given with Apomorphine?

Accepted Answer

Ondansetron

Answer

Dopamine agonist

Answer

Spironolactone

Answer

Aspirin

Question 9

A patient stabilized on warfarin for DVT is given phenobarbitone. Which of the following is the likely interaction between the two drugs?

Accepted Answer

Phenobarbitone increases the metabolism of warfarin

Answer

Phenobarbitone increases the absorption of warfarin

Answer

Phenobarbitone displaces warfarin from plasma protein binding sites

Answer

All of the above

Question 10

Which drug does not result in theophylline toxicity?

Accepted Answer

Amoxicillin

Answer

Ciprofloxacin

Answer

Erythromycin

Answer

Cimetidine

Drug Interactions — MCQs

Drug Interactions — MCQs

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