Drug Interactions Practice Questions

Q: The dose of Digoxin should be reduced when given along with which medication?

Quinidine. ***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.

Q: Which of the following drugs should not be administered concomitantly with warfarin, as it decreases the effect of oral anticoagulants?

Oral contraceptive. ***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.

Q: All of the following drugs are known to reduce the efficacy of combined oral contraceptives when used together, except:

Penicillin. ***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

Q: The anticoagulant activity of warfarin can be reduced by all of the following except.

Aspirin. ***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.

Q: In a patient taking oral contraceptives, the chance of pregnancy increases after taking any of the following drugs except:

Cimetidine. ***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.

Q: All of the following are risk factors for renal toxicity caused by aminoglycosides EXCEPT:

Simultaneous use of penicillin. ***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.

Question 1

The dose of Digoxin should be reduced when given along with which medication?

Accepted Answer

Quinidine

Answer

Rifampicin

Answer

Indomethacin

Answer

Antacids

Question 2

Which of the following drugs should not be administered concomitantly with warfarin, as it decreases the effect of oral anticoagulants?

Accepted Answer

Oral contraceptive

Answer

Aspirin

Answer

Cimetidine

Answer

Broad-spectrum antibiotic

Question 3

All of the following drugs are known to reduce the efficacy of combined oral contraceptives when used together, except:

Accepted Answer

Penicillin

Answer

Rifampin

Answer

Griseofulvin

Answer

Carbamazepine

Question 4

The anticoagulant activity of warfarin can be reduced by all of the following except.

Accepted Answer

Aspirin

Answer

Rifampin

Answer

Vitamin K

Answer

Carbamazepine

Question 5

In a patient taking oral contraceptives, the chance of pregnancy increases after taking any of the following drugs except:

Accepted Answer

Cimetidine

Answer

Griseofulvin

Answer

Ampicillin

Answer

Phenytoin

Question 6

All of the following are risk factors for renal toxicity caused by aminoglycosides EXCEPT:

Accepted Answer

Simultaneous use of penicillin

Answer

Hypokalemia

Answer

Aminoglycoside administration in recent past

Answer

Elderly patient

Drug Interactions — MCQs

Drug Interactions — MCQs

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