Drug Interactions — MCQs
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The dose of Digoxin should be reduced when given along with which medication?
Which of the following drugs should not be administered concomitantly with warfarin, as it decreases the effect of oral anticoagulants?
The anticoagulant activity of warfarin can be reduced by all of the following except.
All of the following drugs are known to reduce the efficacy of combined oral contraceptives when used together, except:
In a patient taking oral contraceptives, the chance of pregnancy increases after taking any of the following drugs except:
All of the following are risk factors for renal toxicity caused by aminoglycosides EXCEPT:
Drug Interactions Indian Medical PG Practice Questions and MCQs
Question 181: The dose of Digoxin should be reduced when given along with which medication?
- A. Quinidine (Correct Answer)
- B. Rifampicin
- C. Indomethacin
- D. Antacids
Explanation: ***Quinidine*** - **Quinidine** is known to increase **digoxin plasma concentrations** by inhibiting its renal and non-renal clearance and by displacing it from tissue-binding sites. - This interaction can significantly elevate digoxin levels, necessitating a **dose reduction** [1] to avoid **digoxin toxicity**. *Rifampicin* - **Rifampicin** induces hepatic enzymes, leading to an **increased metabolism** of many drugs, including digoxin. - This interaction would typically *decrease* digoxin levels, potentially requiring an **increase** in digoxin dose, not a reduction. *Indomethacin* - **Indomethacin** and other **NSAIDs** can impair renal function, which *may* indirectly affect digoxin clearance. - However, it does not have a direct, significant interaction with digoxin that would commonly require a primary dose reduction of digoxin for healthy kidneys. *Antacids* - **Antacids** can *reduce the absorption* of some drugs by altering gastric pH or adsorbing the drug. - While they can slightly decrease digoxin absorption if taken concurrently, this effect is usually not significant enough to necessitate a **digoxin dose reduction** in most clinical settings.
Question 182: Which of the following drugs should not be administered concomitantly with warfarin, as it decreases the effect of oral anticoagulants?
- A. Aspirin
- B. Oral contraceptive (Correct Answer)
- C. Cimetidine
- D. Broad-spectrum antibiotic
Explanation: ***Oral contraceptive*** - **Oral contraceptives** contain estrogen, which increases the synthesis of clotting factors (especially factors II, VII, IX, X), thereby **decreasing the anticoagulant effect of warfarin** and increasing the risk of thrombosis. - This interaction necessitates increased monitoring of **INR** and potential **warfarin dose adjustments** when co-administered. *Cimetidine* - **Cimetidine** is a potent inhibitor of **cytochrome P450 enzymes**, which are responsible for warfarin metabolism. - Co-administration with warfarin would **increase** its anticoagulant effect (by reducing warfarin clearance), leading to a higher risk of bleeding, not a decrease. *Aspirin* - **Aspirin** inhibits **platelet aggregation** through irreversible COX-1 inhibition and does not decrease the effect of warfarin. - It can, however, **increase the risk of bleeding** when combined with warfarin due to its antiplatelet properties, but it doesn't reduce warfarin's anticoagulant action. *Broad-spectrum antibiotic* - Many **broad-spectrum antibiotics** can **potentiate** the effect of warfarin by reducing vitamin K-producing gut flora or inhibiting warfarin metabolism. - This interaction typically leads to an **increased INR** and higher bleeding risk, not a decreased anticoagulant effect.
Question 183: The anticoagulant activity of warfarin can be reduced by all of the following except.
- A. Aspirin (Correct Answer)
- B. Rifampin
- C. Vitamin K
- D. Carbamazepine
Explanation: ***Aspirin*** - **Aspirin** does NOT reduce warfarin's anticoagulant activity; instead, it increases the risk of bleeding through a synergistic effect. - Aspirin inhibits platelet aggregation via **cyclooxygenase-1 (COX-1)** inhibition, preventing thromboxane A2 formation, which is a different mechanism from warfarin's inhibition of vitamin K-dependent clotting factors. - When combined with warfarin, aspirin **potentiates** the overall antithrombotic effect and increases bleeding risk. *Carbamazepine* - **Carbamazepine** is a potent inducer of hepatic cytochrome P450 enzymes (CYP2C9, CYP3A4). - By increasing warfarin metabolism, it **reduces** warfarin's plasma concentrations and decreases its anticoagulant effect. - Patients on this combination may require higher warfarin doses to maintain therapeutic INR. *Rifampin* - **Rifampin** is one of the most potent inducers of hepatic cytochrome P450 enzymes (CYP2C9, CYP3A4). - It significantly increases warfarin metabolism, leading to **reduced** plasma concentrations and diminished anticoagulant effect. - This interaction often necessitates substantial increases in warfarin dosage. *Vitamin K* - **Vitamin K** is the direct antagonist of warfarin's mechanism of action. - Warfarin inhibits vitamin K epoxide reductase, preventing the regeneration of active vitamin K needed for synthesis of clotting factors II, VII, IX, and X. - Administration of vitamin K **reverses** warfarin's anticoagulant effect by bypassing the inhibited enzyme and restoring clotting factor production.
Question 184: All of the following drugs are known to reduce the efficacy of combined oral contraceptives when used together, except:
- A. Rifampin
- B. Penicillin (Correct Answer)
- C. Griseofulvin
- D. Carbamazepine
Explanation: ***Correct Option: Penicillin*** - **Penicillin** and other older antibiotics generally do not significantly interact with combined oral contraceptives (COCs) to reduce their efficacy - While some broader-spectrum antibiotics might theoretically affect gut flora and enterohepatic recirculation of estrogen, **penicillin's impact is minimal** and not considered clinically significant for contraception - This is the correct answer because the question asks for the drug that does NOT reduce COC efficacy *Incorrect Option: Rifampin* - **Rifampin** is a potent inducer of **cytochrome P450 enzymes** in the liver, particularly CYP3A4 - This enzyme system is responsible for metabolizing the estrogen and progestin components of combined oral contraceptives, leading to **faster breakdown** and **reduced contraceptive efficacy** - Backup contraception is recommended when using rifampin with COCs *Incorrect Option: Griseofulvin* - **Griseofulvin**, an antifungal medication, is also an **enzyme inducer**, specifically of cytochrome P450 enzymes - This induction accelerates the metabolism of estrogen and progestin in COCs, thereby **lowering their plasma concentrations** and increasing the risk of contraceptive failure - Alternative contraceptive methods should be considered during griseofulvin therapy *Incorrect Option: Carbamazepine* - **Carbamazepine**, an anticonvulsant, is a strong inducer of hepatic **cytochrome P450 enzymes**, similar to rifampin - Its enzyme-inducing effects lead to **increased metabolism** of the hormonal components of COCs, resulting in **decreased contraceptive effectiveness** - Women on carbamazepine should use alternative or additional contraceptive methods
Question 185: In a patient taking oral contraceptives, the chance of pregnancy increases after taking any of the following drugs except:
- A. Griseofulvin
- B. Ampicillin
- C. Phenytoin
- D. Cimetidine (Correct Answer)
Explanation: ***Cimetidine*** - **Cimetidine** is an H2-receptor antagonist that does not significantly interfere with the metabolism of oral contraceptives, so it is less likely to increase the risk of pregnancy. - It works by reducing stomach acid production and is not a known enzyme inducer or gut flora disturber in a way that would compromise contraceptive efficacy. *Phenytoin* - **Phenytoin** is a potent inducer of **cytochrome P450 enzymes** in the liver, which significantly increases the metabolism of steroid hormones found in oral contraceptives. - Accelerated metabolism leads to lower systemic levels of contraceptive hormones, reducing their efficacy and increasing the risk of unintended pregnancy. *Griseofulvin* - **Griseofulvin** induces **hepatic microsomal enzymes**, primarily cytochrome P450, thereby accelerating the metabolism of contraceptive steroids. - This increases the clearance of contraceptive hormones, diminishing their effectiveness and raising the risk of breakthrough bleeding and pregnancy. *Ampicillin* - **Ampicillin**, like other broad-spectrum antibiotics, has been historically thought to alter the **normal gut flora** responsible for enterohepatic recirculation of estrogen metabolites. - By disrupting this process, ampicillin was believed to reduce the reabsorption of estrogen, leading to lower active hormone levels and potentially decreased contraceptive efficacy. - **Note:** Current evidence suggests this interaction is minimal and clinically insignificant for most antibiotics including ampicillin, though it may still appear in exam questions based on traditional teaching.
Question 186: All of the following are risk factors for renal toxicity caused by aminoglycosides EXCEPT:
- A. Hypokalemia
- B. Aminoglycoside administration in recent past
- C. Simultaneous use of penicillin (Correct Answer)
- D. Elderly patient
Explanation: ***Simultaneous use of penicillin*** - Penicillins are **not considered a major risk factor** for aminoglycoside nephrotoxicity. - While aminoglycosides and penicillins can be inactivated when mixed **in vitro** (in IV solutions), this does not translate to a protective effect against renal toxicity **in vivo**. - The major nephrotoxic drug combinations with aminoglycosides include **vancomycin, amphotericin B, cyclosporine, NSAIDs, and loop diuretics** - not penicillins. *Hypokalemia* - **Electrolyte imbalances**, such as hypokalemia, can worsen renal function and increase the susceptibility of the kidneys to damage from nephrotoxic drugs like aminoglycosides. - **Volume depletion** and electrolyte disturbances are common comorbidities that exacerbate aminoglycoside-induced acute kidney injury. *Aminoglycoside administration in recent past* - Prior exposure to aminoglycosides, especially within a short period, can lead to **cumulative toxicity** due to incomplete renal recovery from previous dosing. - The kidneys require time to regenerate epithelial cells damaged by aminoglycosides, and repeated exposure increases the risk of **irreversible damage**. *Elderly patient* - **Age** is a significant risk factor because elderly patients often have **decreased renal blood flow** and a reduced number of functional nephrons. - The **glomerular filtration rate (GFR)** naturally declines with age, making the kidneys more vulnerable to drug-induced injury.
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