Drug Interactions — MCQs

Drug Interactions Indian Medical PG Practice Questions and MCQs

Question 81: Ciprofloxacin should not be used with theophylline because:

A. it decreases the efficiency of theophylline
B. it increases the toxicity of theophylline (Correct Answer)
C. it decreases the efficiency of ciprofloxacin
D. it decreases the absorption of theophylline

Explanation: The correct answer is **B. it increases the toxicity of theophylline.** **Mechanism:** Ciprofloxacin is a potent inhibitor of the hepatic microsomal enzyme **CYP1A2**. Theophylline is primarily metabolized by this specific isoenzyme. When ciprofloxacin is co-administered, it inhibits the metabolism of theophylline, leading to decreased clearance and a significant rise in its plasma concentration. Since theophylline has a **narrow therapeutic index**, even a slight increase in serum levels can lead to toxicity, manifesting as severe nausea, vomiting, palpitations, tremors, and potentially fatal seizures or arrhythmias [1]. **Analysis of Incorrect Options:** * **Option A & C:** The interaction is not one of antagonism or reduced efficacy. Instead, it is a pharmacokinetic interaction that enhances the drug's effect to a dangerous level. * **Option D:** The interaction occurs during the hepatic metabolism phase (elimination), not during the absorption phase in the gastrointestinal tract. **High-Yield Clinical Pearls for NEET-PG:** * **Fluoroquinolones & CYP1A2:** Not all fluoroquinolones inhibit CYP1A2 equally. **Ciprofloxacin** and **Enoxacin** are potent inhibitors, whereas Levofloxacin and Ofloxacin have minimal effect on theophylline levels. * **Other CYP1A2 Inhibitors:** Fluvoxamine and Clarithromycin also increase theophylline toxicity. * **Enzyme Inducers:** Conversely, smoking and Rifampicin induce CYP1A2, which *decreases* theophylline levels (requiring a dose increase). * **Antacid Interaction:** Remember that oral absorption of Ciprofloxacin itself is decreased by antacids (chelation with Al, Mg, Ca), but this is unrelated to the theophylline interaction.

Question 82: A patient with temporal lobe epilepsy (complex partial seizures) is experiencing recurrent seizures despite being on carbamazepine. Phenobarbital is added as a second drug, but the seizures increase in frequency. What is the probable reason for the apparent adverse effect of adding phenobarbital?

A. Intracerebral bleeding due to worsening bone marrow suppression
B. Decreased carbamazepine level (Correct Answer)
C. Decreased stability of CNS neuronal membranes
D. Hypokalemia

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The primary mechanism behind this interaction is **Enzyme Induction**. Both Carbamazepine and Phenobarbital are potent inducers of the hepatic cytochrome P450 system (specifically **CYP3A4**). When Phenobarbital is added to a stable regimen of Carbamazepine, it accelerates the metabolism of Carbamazepine. This leads to a significant **decrease in the plasma concentration** of Carbamazepine, falling below the therapeutic window and resulting in a loss of seizure control (breakthrough seizures). **2. Why the Incorrect Options are Wrong:** * **Option A:** While Carbamazepine can cause bone marrow suppression (aplastic anemia), this typically leads to infections or bleeding diathesis, not an immediate increase in seizure frequency. * **Option C:** Both drugs are anticonvulsants that *increase* the stability of neuronal membranes (Carbamazepine via sodium channel blockade; Phenobarbital via GABA-A receptor facilitation). They do not destabilize membranes. * **Option D:** Hypokalemia is not a recognized side effect of these anticonvulsants. Carbamazepine is more famously associated with **hyponatremia** (SIADH-like effect). **3. Clinical Pearls for NEET-PG:** * **Auto-induction:** Carbamazepine is unique because it induces its own metabolism (*auto-induction*), meaning doses often need adjustment after the first few weeks of therapy. * **Enzyme Inducers (Mnemonic: GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbital. * **Drug of Choice:** Carbamazepine remains a first-line agent for Focal (Partial) seizures and Trigeminal Neuralgia. * **Therapeutic Drug Monitoring (TDM):** This scenario highlights the importance of TDM when combining multiple anti-epileptic drugs (AEDs) to manage drug-drug interactions.

Question 83: Which of the following drugs should be avoided with warfarin?

A. Antacids
B. Benzodiazepines
C. NSAIDs like Ibuprofen (Correct Answer)
D. Codeine, dihydrocodeine, paracetamol

Explanation: **Explanation:** The interaction between **Warfarin and NSAIDs (like Ibuprofen)** is a high-yield clinical scenario due to the significantly increased risk of life-threatening hemorrhage. This interaction is **pharmacodynamic** in nature: 1. **Antiplatelet Effect:** NSAIDs inhibit COX-1, leading to decreased Thromboxane A2 synthesis, which impairs platelet aggregation. 2. **Mucosal Injury:** NSAIDs inhibit protective prostaglandins in the gastric mucosa, increasing the risk of GI ulceration and bleeding. When combined with Warfarin’s anticoagulant effect (inhibition of Vitamin K epoxide reductase), the body’s primary and secondary hemostatic mechanisms are both compromised. **Analysis of Incorrect Options:** * **Antacids (A):** Generally do not have a clinically significant interaction with warfarin absorption or metabolism. * **Benzodiazepines (B):** These are considered safe to use with warfarin as they do not induce or inhibit the CYP450 enzymes responsible for warfarin metabolism (primarily CYP2C9). * **Codeine/Paracetamol (D):** These are the preferred analgesics for patients on anticoagulants. While high-dose, chronic paracetamol can occasionally elevate INR, it is significantly safer than NSAIDs. **NEET-PG High-Yield Pearls:** * **CYP2C9 Inhibitors:** Drugs like **Amiodarone, Metronidazole, and Erythromycin** increase warfarin levels (elevated INR). * **CYP Enzyme Inducers:** **Rifampicin, Phenytoin, and Carbamazepine** decrease warfarin efficacy (low INR). * **Monitoring:** Warfarin therapy is monitored using **PT/INR** (Target: 2.0–3.0). * **Antidote:** For immediate reversal, use **Prothrombin Complex Concentrate (PCC)** or Fresh Frozen Plasma; for non-emergent reversal, use **Vitamin K1**.

Question 84: Which of the following medications can lead to oral contraceptive failure?

A. Rifampicin (Correct Answer)
B. Cimetidine
C. Propranolol
D. Ethambutol

Explanation: **Explanation:** The correct answer is **Rifampicin (Option A)**. **Mechanism of Interaction:** Oral contraceptive pills (OCPs) are primarily metabolized in the liver by the **Cytochrome P450 (CYP3A4)** enzyme system. Rifampicin is a potent **microsomal enzyme inducer**. It increases the synthesis and activity of these hepatic enzymes, leading to the rapid metabolism and clearance of estrogen and progesterone. This reduces the plasma concentration of the hormones below the therapeutic threshold required to suppress ovulation, resulting in contraceptive failure and unintended pregnancy. **Analysis of Incorrect Options:** * **B. Cimetidine:** This is a known enzyme **inhibitor**. It would decrease the metabolism of OCPs, potentially increasing their plasma levels (and side effects) rather than causing failure. * **C. Propranolol:** This is a non-selective beta-blocker. While it undergoes hepatic metabolism, it does not significantly induce or inhibit the CYP450 enzymes responsible for OCP metabolism. * **D. Ethambutol:** Unlike Rifampicin, Ethambutol is an antitubercular drug that is primarily excreted by the kidneys and does not possess enzyme-inducing properties. **High-Yield Clinical Pearls for NEET-PG:** * **The "GPRS Cell Phone" Mnemonic:** Common enzyme inducers include **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, and **P**henobarbitone. * **Antibiotic Myth:** While Rifampicin is a proven cause of OCP failure, most broad-spectrum antibiotics (like Amoxicillin) do not significantly reduce OCP efficacy in clinical trials, though they were historically thought to do so via alteration of gut flora. * **Clinical Advice:** Patients on Rifampicin should be advised to use an alternative or additional method of contraception (e.g., barrier methods) during and for 28 days after stopping the drug.

Question 85: A woman who has been taking oral contraceptives for several years is diagnosed with epilepsy and started on phenytoin. Which of the following is the most likely consequence of adding phenytoin?

A. Agranulocytosis
B. Phenytoin toxicity
C. Reduced contraceptive efficacy (Correct Answer)
D. Thromboembolism

Explanation: **Explanation:** The correct answer is **Reduced contraceptive efficacy**. This interaction is a classic example of **Cytochrome P450 (CYP450) enzyme induction**. **Mechanism:** Phenytoin is a potent inducer of hepatic microsomal enzymes, specifically the CYP3A4 isoenzyme. Oral contraceptive pills (OCPs) contain estrogen and progesterone, which are metabolized by these same enzymes. When phenytoin is added, it accelerates the metabolism of the hormones in the OCP, leading to sub-therapeutic plasma levels. This can result in breakthrough ovulation and unintended pregnancy. **Analysis of Incorrect Options:** * **Agranulocytosis:** While some anticonvulsants (like Carbamazepine) are associated with blood dyscrasias, this is not a result of an interaction with OCPs. * **Phenytoin toxicity:** OCPs do not significantly inhibit phenytoin metabolism; therefore, toxicity is unlikely. In fact, if an enzyme *inhibitor* (like Cimetidine or Valproate) were added, phenytoin toxicity would be the concern. * **Thromboembolism:** While OCPs themselves increase the risk of thromboembolism, adding phenytoin decreases the concentration of OCPs, theoretically reducing this risk rather than increasing it. **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 Action:** Patients on OCPs starting an enzyme inducer should be advised to use an alternative method of contraception (e.g., barrier methods or an IUD) or a higher dose of estrogen (though the latter is less reliable). * **Rifampicin** is the most potent inducer and a frequent culprit in "OCP failure" questions.

Question 86: A patient is being treated with an antianginal drug. Which of the following antianginal drugs would necessitate avoiding the prescription of Sildenafil?

A. Calcium channel blockers
B. Beta blockers
C. Organic nitrates (Correct Answer)
D. ACE inhibitors

Explanation: The correct answer is Organic nitrates. This is a classic, high-yield drug interaction based on the synergistic activation of the cGMP (cyclic Guanosine Monophosphate) pathway.Mechanism of Interaction:1. Organic Nitrates (e.g., Nitroglycerin, Isosorbide mononitrate) act as nitric oxide (NO) donors. NO stimulates the enzyme Guanylyl Cyclase, which increases the production of cGMP, leading to smooth muscle relaxation and vasodilation [2].2. Sildenafil is a Phosphodiesterase-5 (PDE-5) inhibitor. PDE-5 is the enzyme responsible for breaking down cGMP.3. When taken together, nitrates increase cGMP production while Sildenafil prevents its degradation. This results in a massive, uncontrolled accumulation of cGMP, leading to profound systemic vasodilation and severe, life-threatening hypotension, which can trigger myocardial infarction or stroke [1].Analysis of Incorrect Options: A, B, & D (CCBs, Beta-blockers, ACE inhibitors): While these drugs also lower blood pressure, they do not utilize the NO-cGMP-PDE5 pathway. Although cautious monitoring is required when combining any antihypertensives with Sildenafil, they do not carry the same absolute contraindication as nitrates.NEET-PG High-Yield Pearls: Time Window: Sildenafil should not be taken within 24 hours of nitrate use; for Tadalafil (longer half-life), the window extends to 48 hours [1]. Other PDE-5 Inhibitors: Vardenafil and Tadalafil share this same dangerous interaction with nitrates [1, 2]. Clinical Presentation: If a patient on Sildenafil develops chest pain, nitrates are strictly contraindicated; clinicians should use alternative antianginals like Morphine or Beta-blockers if appropriate [1].

Question 87: The anticoagulant effect of warfarin is increased by all of the following drugs, EXCEPT:

A. Cimetidine
B. Phytonadione (Correct Answer)
C. Amiodarone
D. Phenylbutazone

Explanation: **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.

Question 88: Oral contraceptive pills' efficiency is reduced by the simultaneous use of which of the following medications?

A. Rifampicin and Carbamazepine (Correct Answer)
B. Rifampicin
C. Rifampicin and Tricyclic antidepressants
D. Carbamazepine and Propranolol

Explanation: **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.

Question 89: Which of the following does not affect the metabolism of oral contraceptive pills?

A. Phenytoin
B. Griscofulvin
C. Primidone
D. Penicillin (Correct Answer)

Explanation: **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).

Question 90: Tranylcypromine, a Monoamine Oxidase Inhibitor, should be avoided with which of the following drug classes due to the risk of a dangerous drug interaction?

A. Opioid analgesics
B. Tricyclic antidepressants (Correct Answer)
C. Benzodiazepines
D. All of the above

Explanation: **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.

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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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