Drug Interactions — MCQs

Drug Interactions Indian Medical PG Practice Questions and MCQs

Question 101: Interaction of theophylline with ciprofloxacin is:

A. Ciprofloxacin increases theophylline metabolism
B. Ciprofloxacin decreases theophylline metabolism (Correct Answer)
C. Theophylline increases ciprofloxacin metabolism
D. Theophylline decreases ciprofloxacin metabolism

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The interaction between theophylline and ciprofloxacin is a classic example of **enzyme inhibition**. Theophylline is primarily metabolized in the liver by the cytochrome P450 enzyme **CYP1A2**. Ciprofloxacin is a potent inhibitor of this specific isoenzyme [2]. When co-administered, ciprofloxacin inhibits the metabolism of theophylline, leading to increased plasma concentrations of the drug [1]. Since theophylline has a **narrow therapeutic index**, this inhibition can quickly lead to toxicity, manifesting as nausea, vomiting, tachycardia, arrhythmias, or seizures [1]. **2. Why the Incorrect Options are Wrong:** * **Option A:** Ciprofloxacin is an enzyme inhibitor, not an inducer [2]. Drugs like rifampicin or phenytoin [3] would increase theophylline metabolism. * **Options C & D:** Theophylline does not significantly affect the hepatic metabolism of fluoroquinolones. Ciprofloxacin is primarily eliminated via renal excretion (glomerular filtration and active tubular secretion), making it less susceptible to metabolic drug-drug interactions compared to theophylline. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Cipro-Theophylline" Rule:** Always reduce the dose of theophylline by 30–50% if ciprofloxacin must be used concurrently. * **Other CYP1A2 Inhibitors:** Apart from ciprofloxacin, **fluvoxamine** and **erythromycin** [2] also inhibit theophylline metabolism. * **Fluoroquinolone Exception:** **Levofloxacin** has minimal effect on CYP1A2 and is generally considered a safer alternative for patients on theophylline. * **Smoking Interaction:** Cigarette smoking *induces* CYP1A2, thereby *increasing* theophylline metabolism (the opposite effect of ciprofloxacin).

Question 102: Which of the following drugs should not be administered concomitantly with Sucralfate?

A. Ranitidine (Correct Answer)
B. Ciprofloxacin
C. Propranolol
D. Diphenhydramine

Explanation: **Explanation:** **1. Why Ranitidine is the Correct Answer:** Sucralfate is a complex of aluminum hydroxide and sulfated sucrose used to treat peptic ulcers. Its mechanism of action is **pH-dependent**: in an acidic environment (pH < 4), sucralfate undergoes polymerization to form a sticky, viscous paste that binds to the ulcer base, creating a physical barrier against acid and pepsin. **Ranitidine**, an H2-receptor antagonist, increases gastric pH by inhibiting acid secretion. When administered together, Ranitidine prevents the activation of Sucralfate, significantly reducing its therapeutic efficacy. Therefore, Sucralfate should be taken on an empty stomach, at least 30–60 minutes before meals or acid-suppressing agents. **2. Analysis of Incorrect Options:** * **B. Ciprofloxacin:** While Sucralfate can decrease the *absorption* of fluoroquinolones (like Ciprofloxacin) due to chelation by aluminum ions, it does not interfere with the mechanism of Sucralfate itself. This interaction is managed by spacing the doses. * **C. Propranolol & D. Diphenhydramine:** These drugs do not significantly alter gastric pH or interfere with the polymerization of Sucralfate. While Sucralfate may slightly delay the absorption of various drugs due to its physical coating effect, it is not contraindicated with these specific agents. **3. NEET-PG High-Yield Pearls:** * **Activation Requirement:** Sucralfate requires an **acidic medium** for activation. Avoid concomitant use with Antacids, H2-blockers, or PPIs. * **The "Band-Aid" Drug:** It is often referred to as a "physical barrier" or "cytoprotective" agent. * **Side Effects:** The most common side effect is **constipation** (due to the aluminum content). * **Chelation:** Like antacids, Sucralfate can bind to and decrease the absorption of Digoxin, Phenytoin, and Tetracyclines. Always advise a 2-hour gap between these medications.

Question 103: A patient with glaucoma develops blepharoconjunctivitis after instilling an antiglaucoma drug. Which of the following drugs can be responsible for this adverse reaction?

A. Timolol (Correct Answer)
B. Latanoprost
C. Dorzolamide
D. Pilocarpine

Explanation: **Explanation:** The correct answer is **Timolol**. **1. Why Timolol is correct:** Timolol is a non-selective beta-blocker and the most commonly used topical agent for glaucoma. **Blepharoconjunctivitis** (inflammation of the eyelids and conjunctiva) is a well-documented local adverse effect of chronic topical beta-blocker therapy. This reaction is often a form of **Type IV hypersensitivity (allergic contact dermatitis)** triggered either by the drug itself or the preservative (commonly Benzalkonium chloride) used in the formulation. Patients typically present with itching, redness, and crusting of the eyelids. **2. Why other options are incorrect:** * **Latanoprost:** Being a Prostaglandin F2α analog, its hallmark side effects include increased iris pigmentation (heterochromia), thickening/darkening of eyelashes (trichomegaly), and conjunctival hyperemia, but it is less commonly associated with classic blepharoconjunctivitis compared to Timolol. * **Dorzolamide:** As a topical Carbonic Anhydrase Inhibitor, its most specific side effects are a bitter taste (dysgeusia) and stinging/burning upon instillation. While it can cause local irritation, Timolol is the classic association for blepharoconjunctivitis in exam vignettes. * **Pilocarpine:** A miotic (cholinergic agonist) primarily associated with "brow ache," pupillary constriction (miosis), and accommodative spasm. **3. High-Yield Clinical Pearls for NEET-PG:** * **Timolol Contraindications:** Always screen for Bronchial Asthma and Bradycardia/Heart block, as systemic absorption through the nasolacrimal duct can occur. * **Preservative-Free Drops:** If a patient develops blepharoconjunctivitis, switching to preservative-free formulations is the first line of management. * **Latanoprost Fact:** It is the drug of choice for Open-Angle Glaucoma (OAG) due to its once-daily dosing and superior efficacy in increasing uveoscleral outflow.

Question 104: Combined oral contraceptives interfere with the action of which of the following drugs, except?

A. Imipramine
B. Meperidine
C. Metoprolol
D. Metformin (Correct Answer)

Explanation: **Explanation:** The core concept behind this question is the effect of **Combined Oral Contraceptive Pills (COCPs)** on the **Cytochrome P450 (CYP)** enzyme system. Estrogens and progestins in COCPs act as **enzyme inhibitors**, specifically inhibiting the metabolism of several drugs, leading to increased plasma levels and potential toxicity. **1. Why Metformin is the Correct Answer:** Metformin is a biguanide used in Type 2 Diabetes. Unlike many other drugs, Metformin is **not metabolized by the liver** or the CYP450 system. It is excreted **unchanged in the urine** via organic cation transporters (OCT2). Therefore, COCPs do not interfere with its metabolic clearance. While COCPs can theoretically decrease glucose tolerance (pharmacodynamic antagonism), they do not interfere with the "action" or metabolism of Metformin in the same inhibitory way they do with the other listed drugs. **2. Why the other options are incorrect:** * **Imipramine (TCA):** COCPs inhibit the oxidative metabolism of Tricyclic Antidepressants, leading to increased plasma concentrations and risk of anticholinergic toxicity. * **Meperidine (Opioid):** COCPs inhibit the metabolism of Meperidine (Pethidine), potentially leading to increased sedation and respiratory depression. * **Metoprolol (Beta-blocker):** COCPs inhibit the CYP2D6-mediated metabolism of Metoprolol, which can result in enhanced bradycardia or hypotension. **High-Yield Clinical Pearls for NEET-PG:** * **COCPs as Inhibitors:** They increase levels of Cyclosporine, Theophylline, and Corticosteroids. * **COCPs as Inducers:** Interestingly, they can *induce* glucuronidation, potentially **decreasing** levels of drugs like **Lamotrigine** and **Lorazepam**. * **The Reverse Interaction:** Remember that **Enzyme Inducers** (Rifampicin, Phenytoin, Carbamazepine) decrease the efficacy of COCPs, leading to **contraceptive failure**. This is a more common clinical scenario tested in exams.

Question 105: Oral anticoagulants given to pregnant women cause which of the following fetal malformations?

A. Long bones limb defect
B. Craniofacial malformation (Correct Answer)
C. Cardiovascular malformation
D. Costochondrodysplasia

Explanation: **Explanation:** The use of oral anticoagulants, specifically **Warfarin**, during the first trimester of pregnancy (6th–9th week) leads to a condition known as **Fetal Warfarin Syndrome (Warfarin Embryopathy)**. Warfarin crosses the placenta and inhibits the γ-carboxylation of osteocalcin and other bone proteins, leading to defective calcification. **Why Option B is Correct:** The hallmark of Fetal Warfarin Syndrome is **Craniofacial malformations**. This typically includes **midfacial hypoplasia** (depressed nasal bridge) and **choanal atresia**. These features occur because the drug interferes with the development of the cartilaginous structures of the face and skull during the critical period of organogenesis. **Analysis of Incorrect Options:** * **A. Long bones limb defect:** While Warfarin can cause stippled epiphyses (Chondrodysplasia punctata), it does not typically cause gross long-bone limb reduction defects (which are more characteristic of Thalidomide). * **C. Cardiovascular malformation:** These are more commonly associated with drugs like Lithium (Ebstein’s anomaly) or ACE inhibitors, rather than being a primary feature of Warfarin embryopathy. * **D. Costochondrodysplasia:** This is a generic term for cartilage and bone disorders. While Warfarin affects cartilage, the specific NEET-PG clinical presentation focuses on the facial and nasal structures. **High-Yield Clinical Pearls for NEET-PG:** * **Safe Alternative:** Heparin (LMWH or UFH) is the anticoagulant of choice in pregnancy because it is a large polar molecule that **does not cross the placenta**. * **Critical Period:** Exposure between **6–9 weeks** gestation is most hazardous for Warfarin embryopathy. * **Late Pregnancy Risk:** Use in the third trimester can lead to fetal CNS hemorrhage and microcephaly due to the immature fetal liver's inability to synthesize clotting factors. * **Key Triad:** Nasal hypoplasia, stippled epiphyses, and CNS defects.

Question 106: Oral contraceptive failure can occur with which of the following medications?

A. Ketoconazole
B. Aminoglutethimide
C. Metyrapone
D. Glucocorticoids (Correct Answer)

Explanation: **Explanation:** The correct answer is **Glucocorticoids**. **Mechanism of Interaction:** Oral contraceptive pills (OCPs) are primarily metabolized by the hepatic cytochrome P450 enzyme system, specifically the **CYP3A4** isoenzyme. Glucocorticoids (such as dexamethasone or prednisolone) act as **enzyme inducers**. By inducing CYP3A4, they accelerate the metabolism of the estrogenic and progestogenic components of the OCP. This leads to decreased plasma concentrations of the hormones, potentially falling below the therapeutic threshold required to suppress ovulation, thereby resulting in contraceptive failure. **Analysis of Incorrect Options:** * **Ketoconazole:** This is a potent **enzyme inhibitor** (specifically CYP3A4). It would likely increase the plasma levels of OCPs, potentially increasing side effects, but it does not cause contraceptive failure. * **Aminoglutethimide:** While it is an enzyme inducer, its primary clinical use is to inhibit adrenal steroid synthesis (medical adrenalectomy). In the context of standard NEET-PG pharmacology, glucocorticoids are the more classic example of drugs affecting OCP efficacy in this list. * **Metyrapone:** This drug inhibits 11-beta-hydroxylase to interfere with cortisol synthesis. It is not a significant hepatic enzyme inducer and does not typically lead to OCP failure. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Cause:** The most notorious drug causing OCP failure is **Rifampicin** (potent CYP inducer). * **Other Inducers:** Antiepileptics (Phenytoin, Carbamazepine, Phenobarbitone) and Griseofulvin are frequent culprits. * **Antibiotic Myth:** While broad-spectrum antibiotics (like Ampicillin) were historically thought to cause failure by disrupting enterohepatic circulation, current evidence suggests this is clinically insignificant for most patients, unlike enzyme inducers. * **Management:** Patients on enzyme-inducing drugs should be advised to use an alternative or additional method of contraception (e.g., barrier methods).

Question 107: All of the following cause inhibition of CYP3A except?

A. Saquinavir (Correct Answer)
B. Ritonavir
C. Itraconazole
D. Erythromycin

Explanation: **Explanation:** The Cytochrome P450 (CYP) enzyme system, specifically the **CYP3A4** isoform, is responsible for the metabolism of over 50% of clinically used drugs. Understanding its inhibitors and inducers is a high-yield topic for NEET-PG. **Why Saquinavir is the correct answer:** While Saquinavir is a Protease Inhibitor (PI) used in HIV treatment, it is primarily a **substrate** of CYP3A4 rather than a potent inhibitor. In clinical practice, Saquinavir has poor bioavailability because it is extensively metabolized by CYP3A4. To counter this, it is often "boosted" by co-administration with Ritonavir. **Analysis of Incorrect Options:** * **Ritonavir (Option B):** This is the **most potent inhibitor** of CYP3A4 among the Protease Inhibitors. It is frequently used in "pharmacokinetic boosting" to increase the plasma concentrations of other PIs (like Saquinavir or Lopinavir) by inhibiting their metabolism. * **Itraconazole (Option C):** Azole antifungals (especially Ketoconazole and Itraconazole) are classic, potent inhibitors of CYP3A4. They interfere with the synthesis of fungal ergosterol but also cross-react with human CYP enzymes. * **Erythromycin (Option D):** This macrolide antibiotic is a well-known CYP3A4 inhibitor. It can lead to dangerous drug interactions, such as increasing levels of Statins (causing rhabdomyolysis) or Theophylline (causing toxicity). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for CYP3A4 Inhibitors (VITAMIN G):** **V**erapamil, **I**traconazole (and other azoles), **T**elithromycin/Amiodarone, **A**miodarone, **M**acrolides (except Azithromycin), **I**ndinavir/**R**itonavir, **N**efazodone, **G**rapefruit juice. * **Note:** Among macrolides, **Azithromycin** does not inhibit CYP enzymes, making it safer regarding drug interactions. * **Note:** Among PIs, **Ritonavir** is the strongest inhibitor, while **Saquinavir** is the weakest.

Question 108: Dextromethorphan should not be administered concurrently with which class of drugs?

A. Selective Serotonin Reuptake Inhibitors (SSRIs)
B. Monoamine Oxidase Inhibitors (MAOIs) (Correct Answer)
C. Atropine
D. Paracetamol

Explanation: **Explanation:** The correct answer is **Monoamine Oxidase Inhibitors (MAOIs)**. **1. Why MAOIs are the correct answer:** Dextromethorphan is a commonly used antitussive (cough suppressant) that acts as a weak **serotonin reuptake inhibitor**. When administered concurrently with MAOIs (such as Phenelzine or Selegiline), there is a dangerous accumulation of serotonin in the synaptic cleft. This occurs because MAOIs prevent the breakdown of serotonin, while dextromethorphan prevents its reuptake. This interaction can precipitate **Serotonin Syndrome**, a potentially fatal condition characterized by hyperthermia, muscle rigidity, autonomic instability, and mental status changes. **2. Why the other options are incorrect:** * **SSRIs (Option A):** While SSRIs also increase serotonin levels and carry a theoretical risk of serotonin syndrome when combined with dextromethorphan, the interaction with **MAOIs** is significantly more severe and is considered a classic, absolute contraindication in pharmacology. * **Atropine (Option C):** Atropine is an anticholinergic. While dextromethorphan may have mild anticholinergic effects at very high doses, there is no major clinically significant contraindication between the two. * **Paracetamol (Option D):** Paracetamol is frequently combined with dextromethorphan in multi-ingredient over-the-counter "cold and flu" preparations. There is no adverse interaction between them. **3. NEET-PG High-Yield Clinical Pearls:** * **The "Two-Week Rule":** Dextromethorphan should not be used within 14 days of discontinuing an MAOI to allow for enzyme regeneration. * **Dextromethorphan Mechanism:** It is the d-isomer of the codeine analog levorphanol. Unlike opioids, it does not have significant analgesic or addictive properties at standard doses but acts on NMDA receptors and sigma-1 receptors. * **Other Serotonin Syndrome Triggers:** Be wary of combining Linezolid (an antibiotic with MAOI activity), Tramadol, or Pethidine with other serotonergic drugs.

Question 109: Clinically significant drug interaction occurs between pyridoxine and all the following drugs except?

A. Isoniazid
B. Cyclosporine (Correct Answer)
C. Levodopa
D. Hydralazine

Explanation: **Explanation:** The core concept behind this question is the metabolic relationship between certain drugs and **Vitamin B6 (Pyridoxine)**. Pyridoxine acts as a co-factor for various decarboxylase enzymes. **Why Cyclosporine is the correct answer:** Cyclosporine is a calcineurin inhibitor used as an immunosuppressant. Its metabolism and mechanism of action do not involve pyridoxine pathways, nor does it interfere with B6 absorption or excretion. Therefore, there is no clinically significant interaction between the two. **Why the other options are incorrect:** * **Isoniazid (INH):** This is a classic interaction. INH inhibits the enzyme *pyridoxine phosphokinase*, preventing the conversion of B6 to its active form (Pyridoxal Phosphate). It also forms hydrazones with B6, leading to its excretion. This causes **peripheral neuropathy**, which is why B6 (10–50 mg/day) is co-administered with INH. * **Levodopa:** Pyridoxine is a cofactor for the enzyme *peripheral dopa-decarboxylase*. High doses of B6 accelerate the peripheral conversion of Levodopa to Dopamine. Since Dopamine cannot cross the blood-brain barrier, this **reduces the therapeutic efficacy** of Levodopa in Parkinsonism. (Note: This interaction is minimized when Levodopa is combined with Carbidopa). * **Hydralazine:** Similar to INH, this vasodilator can react with pyridoxine to form complexes, leading to a deficiency that manifests as peripheral neuritis. **High-Yield Clinical Pearls for NEET-PG:** * **Penicillamine** also causes B6 deficiency by forming a thiazolidine derivative with the vitamin. * **Oral Contraceptive Pills (OCPs)** can induce a relative B6 deficiency, sometimes linked to depression in users. * **Sideroblastic Anemia:** Pyridoxine is used to treat certain types of sideroblastic anemia as it is a cofactor for ALA synthase.

Question 110: Pethidine should not be given with which of the following drug classes?

A. Reserpine
B. Propranolol
C. Atenolol
D. MAO inhibitors (Correct Answer)

Explanation: **Explanation:** The contraindication of **Pethidine (Meperidine)** with **MAO Inhibitors (MAOIs)** is a classic, high-yield pharmacological interaction. **Why MAO Inhibitors are the Correct Answer:** Pethidine acts as a weak Serotonin Reuptake Inhibitor (SRI). When combined with MAOIs (which prevent the breakdown of serotonin), it can lead to a life-threatening **Serotonin Syndrome**. This manifests as the "Excitatory Type" reaction, characterized by hyperpyrexia (very high fever), agitation, tremors, convulsions, and cardiovascular instability. Additionally, a "Depressive Type" reaction can occur, involving respiratory depression and hypotension, due to the inhibition of hepatic enzymes by MAOIs, which leads to toxic accumulation of pethidine. **Why Other Options are Incorrect:** * **Reserpine (A):** While reserpine depletes monoamines, it does not trigger the acute, fatal serotonergic storm seen with MAOIs. * **Propranolol (B) & Atenolol (C):** These are Beta-blockers. While they may have minor interactions with various drugs regarding heart rate or blood pressure, there is no absolute contraindication or major toxic syndrome associated with their co-administration with pethidine. **Clinical Pearls for NEET-PG:** * **The "Pethidine Rule":** Always screen for MAOI use (e.g., Phenelzine, Selegiline) before administering pethidine. * **Alternative:** If an opioid is needed for a patient on MAOIs, **Morphine** is generally considered safer as it lacks significant serotonergic activity. * **Metabolite Fact:** Pethidine is metabolized to **Norpethidine**, which is neurotoxic and can cause seizures, especially in patients with renal failure. * **Unique Property:** Unlike morphine, pethidine has **anticholinergic properties**, leading to side effects like tachycardia and mydriasis (dilated pupils).

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Mechanisms of Drug Interactions

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Pharmacokinetic Interactions

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Pharmacodynamic Interactions

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Drug-Disease Interactions

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Clinically Significant Drug Interactions

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Computer Systems for Detecting Drug Interactions

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Management of Drug Interactions

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Drug Interactions in Special Populations

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Role of P-glycoprotein in Drug Interactions

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