Drug Interactions — MCQs

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: 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: 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.

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Clinically Significant Drug Interactions

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