Drug Interactions — MCQs

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.

Practice by Chapter

Mechanisms of Drug Interactions

Practice Questions

Pharmacokinetic Interactions

Practice Questions

Pharmacodynamic Interactions

Practice Questions

Drug-Food Interactions

Practice Questions

Drug-Disease Interactions

Practice Questions

Clinically Significant Drug Interactions

Practice Questions

Computer Systems for Detecting Drug Interactions

Practice Questions

Management of Drug Interactions

Practice Questions

Drug Interactions in Special Populations

Practice Questions

Role of P-glycoprotein in Drug Interactions

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Scan to download app